ALBERT

Description: The sensitivity of global climate to the episodicity of fire aerosol emissions Atmospheric Chemistry and Physics Discussions, 13, 23691-23717, 2013 Author(s): S. K. Clark, D. S. Ward, and N. M. Mahowald One of the major ways in which forest and grass fires have an impact on global climate is through the release of aerosols. Most studies focusing on calculating the radiative forcing and other climate impacts of fire aerosols use monthly mean emissions derived from the Global Fire Emissions Database that captures only the seasonal cycle of fire aerosol emissions. Here we present the results of a sensitivity study that investigates the climate response to the episodicity of the fires, based on the standard approach which releases emissions every day, and contrasts that to the response when fires are represented as intense pulses of emissions that occur only over 1–2 days on a monthly, yearly, or five-yearly basis. Overall we find that in the modified cases with increased levels of episodicity, the all sky direct effect radiative forcing increases, the clear sky direct effect radiative forcing remains relatively constant, and the magnitude of the indirect effect radiative forcing decreases by about 1 W m −2 (from −1.6 to −0.6 W m −2 ). In the long term, we find that an increase in aerosol emission episodicity leads to an asymmetric change in indirect radiative forcing in the Northern Hemisphere compared to the Southern Hemisphere contributes to a slight shift in the annual average position of the intertropical convergence zone (ITCZ). This shift is found to have a mixed effect on the overall performance of the model at predicting precipitation rates in the tropics. Given these results we conclude that future studies that look to assess the present day global climate impacts of fire aerosols should consider the need to accurately represent fire episodicity.

Description: A new data set of soil mineralogy for dust-cycle modeling Atmospheric Chemistry and Physics Discussions, 13, 23943-23993, 2013 Author(s): E. Journet, Y. Balkanski, and S. P. Harrison The mineralogy of airborne dust affects the impact of dust particles on direct and indirect radiative forcing, on atmospheric chemistry and on biogeochemical cycling. It is determined partly by the mineralogy of the dust-source regions and partly by size-dependent fractionation during erosion and transport. Here we present a data set that characterizes the clay and silt sized fractions of global soil units in terms of the abundance of 12 minerals that are important for dust-climate interactions: quartz, feldspars, illite, smectite, kaolinite, chlorite, vermiculite, mica, calcite, gypsum, hematite and goethite. The basic mineralogical information is derived from the literature, and is then expanded following explicit rules, in order to characterize as many soil units as possible. We present three alternative realisations of the mineralogical maps that account for the uncertainties in the mineralogical data. We examine the implications of the new database for calculations of the single scattering albedo of airborne dust and thus for dust radiative forcing.

Description: Growth of sulphuric acid nanoparticles under wet and dry conditions Atmospheric Chemistry and Physics Discussions, 13, 24087-24125, 2013 Author(s): L. Škrabalová, D. Brus, T. Anttila, V. Ždímal, and H. Lihavainen New particle formation, which greatly influences the number concentrations and size distributions of an atmospheric aerosol, is often followed by a rapid growth of freshly formed particles. The initial growth of a newly formed aerosol is the crucial process determining the fraction of nucleated particles growing into cloud condensation nuclei sizes, which have a significant influence on climate. In this study, we report the laboratory observations of the growth of nanoparticles produced by nucleation of H 2 SO 4 and water in a laminar flow tube at temperatures of 283, 293 and 303 K, under dry (a relative humidity of 1%) and wet conditions (relative humidity of 30%) and residence times of 30, 45, 60 and 90 s. The initial H 2 SO 4 concentration spans the range from 2 × 10 8 to 1.4 × 10 10 molecule cm −3 and the calculated wall losses of H 2 SO 4 were assumed to be diffusion limited. The detected particle number concentrations, measured by the Ultrafine Condensation Particle Counter (UCPC) and Differential Mobility Particle Sizer (DMPS), were found to depend strongly on the residence time. Hygroscopic particle growth, presented by growth factors, was found to be in good agreement with the previously reported studies. The experimental growth rates ranged from 20 nm h −1 to 890 nm h −1 at RH 1% and from 7 nm h −1 to 980 nm h −1 at RH 30% and were found to increase significantly with the increasing concentration of H 2 SO 4 . Increases in the nucleation temperature had a slight enhancing effect on the growth rates under dry conditions. The influence of relative humidity on growth was not consistent – at lower H 2 SO 4 concentrations, the growth rates were higher under dry conditions while at H 2 SO 4 concentrations greater than 1×10 9 molecule cm −3 the growth rates were higher under wet conditions. The growth rates show only a weak dependence on the residence time. The experimental observations were compared with predictions made using a numerical model, which investigates the growth of particles with three different extents of neutralization by the ammonia NH 3 : (1) pure H 2 SO 4 – H 2 O particles (2) particles formed by ammonium bisulphate, (NH 4 )HSO 4 (3) particles formed by ammonium sulphate, (NH 4 ) 2 SO 4 . The highest growth rates were found for ammonium sulphate particles. Since the model accounting for the initial H 2 SO 4 concentration predicted the experimental growth rates correctly, our results suggest that the commonly presumed diffusional wall losses of H 2 SO 4 are not so significant. We therefore assume that there are not only losses of H 2 SO 4 on the wall but also a flux of H 2 SO 4 molecules from the wall into the flow tube, the effect being more profound under dry conditions and at higher temperatures of the tube wall. Based on a comparison with the atmospheric observations, our results indicate that sulphuric acid alone can not explain the growth rates of particles formed in the atmosphere.

Description: Impacts of different plant functional types on ambient ozone predictions in the Seoul Metropolitan Areas (SMA), Korea Atmospheric Chemistry and Physics Discussions, 13, 24925-24973, 2013 Author(s): H.-K. Kim, J.-H. Woo, R. S. Park, C. H. Song, J.-H. Kim, S.-J. Ban, and J.-H. Park Plant functional type (PFT) distributions affect the results of biogenic emission modeling as well as O 3 and PM simulations using chemistry-transport models (CTMs). This paper analyzes the variations of both surface biogenic VOC emissions and O 3 concentrations due to changes in the PFT distributions in the Seoul Metropolitan Areas, Korea. Also, this paper attempts to provide important implications for biogenic emissions modeling studies for CTM simulations. MM5-MEGAN-SMOKE-CMAQ model simulations were implemented over the Seoul Metropolitan Areas in Korea to predict surface O 3 concentrations for the period of 1 May to 31 June 2008. Starting from MEGAN biogenic emissions analysis with three different sources of PFT input data, US EPA CMAQ O 3 simulation results were evaluated by surface O 3 monitoring datasets and further considered on the basis of geospatial and statistical analyses. The three PFT datasets considered were "(1)KORPFT", developed with a region specific vegetation database; (2) CDP, adopted from US NCAR; and (3) MODIS, reclassified from the NASA Terra and Aqua combined land cover products. Comparisons of MEGAN biogenic emission results with the three different PFT data showed that broadleaf trees (BT) are the most significant contributor, followed by needleleaf trees (NT), shrub (SB), and herbaceous plants (HB) to the total biogenic volatile organic compounds (BVOCs). In addition, isoprene from BT and terpene from NT were recognized as significant primary and secondary BVOC species in terms of BVOC emissions distributions and O 3 -forming potentials in the study domain. Multiple regression analyses with the different PFT data (δO 3 vs. δPFTs) suggest that KORPFT can provide reasonable information to the framework of MEGAN biogenic emissions modeling and CTM O 3 predictions. Analyses of the CMAQ performance statistics suggest that deviations of BT areas can significantly affect CMAQ isoprene and O 3 predictions. From further evaluations of the isoprene and O 3 prediction results, we explored the PFT area-loss artifact that occurs due to geographical disparity between the PFT and leaf area index distributions, and can cause increased bias in CMAQ O 3 . Thus, the PFT-loss artifact must be a source of limitation in the MEGAN biogenic emission modeling and the CTM O 3 simulation results. Time changes of CMAQ O 3 distributions with the different PFT scenarios suggest that hourly and local impacts from the different PFT distributions on occasional inter-deviations of O 3 are quite noticeable, reaching up to 10 ppb. Exponentially diverging hourly BVOC emissions and O 3 concentrations with increasing ambient temperature suggest that the use of representative PFT distributions becomes more critical for O 3 air quality modeling (or forecasting) in support of air quality decision-making and human health studies.

Description: Incidence of rough and irregular atmospheric ice particles from Small Ice Detector 3 measurements Atmospheric Chemistry and Physics Discussions, 13, 24975-25012, 2013 Author(s): Z. Ulanowski, P. H. Kaye, E. Hirst, R. S. Greenaway, R. J. Cotton, E. Hesse, and C. T. Collier The knowledge of properties of ice crystals such as size, shape, concavity and roughness is critical in the context of radiative properties of ice and mixed phase clouds. Limitations of current cloud probes to measure these properties can be circumvented by acquiring two-dimensional light scattering patterns instead of particle images. Such patterns were obtained in situ for the first time using the Small Ice Detector 3 (SID-3) probe during several flights in a variety of mid-latitude mixed phase and cirrus clouds. The patterns are analyzed using several measures of pattern texture, selected to reveal the magnitude of particle roughness or complexity. The retrieved roughness is compared to values obtained from a range of well-characterized test particles in the laboratory. It is found that typical in situ roughness corresponds to that found in the rougher subset of the test particles, and sometimes even extends beyond the most extreme values found in the laboratory. In this study we do not differentiate between small-scale, fine surface roughness and large-scale crystal complexity. Instead, we argue that both can have similar manifestations in terms of light scattering properties and also similar causes. Overall, the in situ data is consistent with ice particles with highly irregular or rough surfaces being dominant. Similar magnitudes of roughness were found in growth and sublimation zones of cirrus. The roughness was found to be negatively correlated with the halo ratio, but not with other thermodynamic or microphysical properties found in situ. Slightly higher roughness was observed in cirrus forming in clean oceanic airmasses than in a continental, polluted one. Overall, the roughness and complexity is expected to lead to increased shortwave cloud reflectivity, in comparison with cirrus composed of more regular, smooth ice crystal shapes. These findings put into question suggestions that climate could be modified through aerosol seeding to reduce cirrus cover and optical depth, as the seeding may result in decreased shortwave reflectivity.

Description: Factors controlling pollutant plume length downwind of major roadways in nocturnal surface inversions Atmospheric Chemistry and Physics Discussions, 13, 25253-25290, 2013 Author(s): W. Choi, A. M. Winer, and S. E. Paulson A curve fit method using a Gaussian dispersion model solution was successfully applied to obtain both dispersion coefficients and a particle number emission factor (PNEF) directly from ultrafine particle (UFP) concentration profiles observed downwind of major roadways in California's South Coast Air Basin (SoCAB). The Briggs' formulation for the vertical dispersion parameter σ z was adopted in this study due to its better performance in describing the observed profiles compared to other formulations examined. The two dispersion coefficients in Briggs' formulation, α and β, ranged from 0.02 to 0.07 and from −0.5 × 10 −3 to 2.8 × 10 −3 , respectively, for the four freeway transects studied and are significantly different for freeways passing over vs. under the street on which measurements of the freeway plume were made. These ranges are wider than literature values for α and β under stable conditions. The dispersion coefficients derived from observations showed strong correlations with both surface meteorology (wind speed/direction, temperature, and air stability) and differences in concentrations between the background and plume peak. The relationships were applied to predict freeway plume transport using a multivariate regression, and produced excellent agreement with observed UFP concentration profiles. The mean PNEF for a mixed vehicle fleet on the four freeways was estimated as 1.2 × 10 14 particles mi −1 vehicle −1 , which is about 15% of the value estimated in 2001 for the I-405 freeway, implying significant reductions in UFP emissions over the past decade in the SoCAB.

Description: Summer Sea Ice Albedo in the Arctic in CMIP5 models Atmospheric Chemistry and Physics Discussions, 13, 25219-25251, 2013 Author(s): T. Koenigk, A. Devasthale, and K.-G. Karlsson Spatial and temporal variations of summer sea ice albedo over the Arctic are analyzed using an ensemble of historical CMIP5 model simulations. The results are compared to the CLARA-SAL product that is based on long-term satellite observations. The summer sea ice albedo varies substantially among CMIP5 models and many models show large biases compared to the CLARA-SAL product. Single summer months show an extreme spread of ice albedo among models; July-values vary between 0.3 and 0.7 for individual models. The CMIP5 ensemble mean, however, agrees relatively well in the Central Arctic but shows too high ice albedo near the ice edges and coasts. In most models, the ice albedo is spatially too uniformly distributed. The summer to summer variations seem to be underestimated in many global models and almost no model is able to fully reproduce the temporal evolution of ice albedo throughout the summer. While the satellite observations indicate the lowest ice albedos during August, the models show minimum values in July and substantially higher values in August. Instead, the June values are often lower in the models than in the satellite observations. This is probably due to too high surface temperatures in June, leading to an early start of the melt season and too cold temperatures in August causing an earlier refreezing in the models. The summer sea ice albedo in the CMIP5 models is strongly governed by surface temperature and snow conditions, particularly during the period of melt onset in early summer and refreezing in late summer. The summer surface net solar radiation of the ice covered Arctic areas is highly related to the ice albedo in the CMIP5 models. However, the impact of the ice albedo on the sea ice conditions in the CMIP5 models is not clearly visible. This indicates the importance of other Arctic and large scale processes for the sea ice conditions.

Description: Intercontinental transport and deposition patterns of atmospheric mercury from anthropogenic emissions Atmospheric Chemistry and Physics Discussions, 13, 25185-25218, 2013 Author(s): L. Chen, H.-H. Wang, J.-F. Liu, W. Zhang, D. Hu, C. Chen, and X.-J. Wang Global policies that regulate anthropogenic mercury emissions to the environment require quantitative and comprehensive source–receptor relationships for mercury emissions, transport and deposition among major continental regions. In this study, we use the GEOS-Chem model to establish source–receptor relationships among eleven major continental regions worldwide. Source–receptor relationships for surface mercury concentrations (SMC) show that some regions (e.g. East Asia, the Indian subcontinent and Europe) should be responsible for their local surface Hg(II) and Hg(P) concentrations because of near-field transport and deposition contributions from their local anthropogenic emissions (up to 64% and 71% for Hg(II) and Hg(P), respectively, over East Asia). We define region of primary influence (RPI) and region of secondary influence (RSI) to establish intercontinental influence patterns. Results indicate that East Asia is SMC RPI for almost all other regions, while Europe, Russia and the Indian subcontinent also make some contributions to SMC over some receptor regions because they are dominant RSI source regions. Source–receptor relationships for mercury deposition show that approximately 16% and 17% of dry and wet deposition, respectively, over North America originate from East Asia, indicating that trans-pacific transport of East Asian emissions is the major foreign source of mercury deposition in North America. Europe, Southeast Asia and the Indian subcontinent are also important mercury deposition sources for some receptor regions because they are dominant RSI. We also quantify seasonal variation on mercury deposition contributions over other regions from East Asia. Results show that mercury deposition (including dry and wet) contributions from East Asia over the Northern Hemisphere receptor regions (e.g. North America, Europe, Russia, Middle East and Middle Asia) vary seasonally, with the maximum values in summer and minimum values in winter. The opposite seasonal pattern occurs on mercury dry deposition contributions over Southeast Asia and the Indian subcontinent.

Description: Balloon-borne match measurements of mid-latitude cirrus clouds Atmospheric Chemistry and Physics Discussions, 13, 25417-25479, 2013 Author(s): A. Cirisan, B. P. Luo, I. Engel, F. G. Wienhold, U. K. Krieger, U. Weers, G. Romanens, G. Levrat, P. Jeannet, D. Ruffieux, R. Philipona, B. Calpini, P. Spichtinger, and T. Peter Observations of persistent high supersaturations with respect to ice inside cirrus clouds are challenging our understanding of cloud microphysics and of climate feedback processes in the upper troposphere. Single measurements of a cloudy air mass provide only a snapshot from which the persistence of ice supersaturation cannot be judged. We introduce here the "cirrus match technique" to obtain information of the evolution of clouds and their saturation ratio. The aim of these coordinated balloon soundings is to analyze the same air mass twice. To this end the standard radiosonde equipment is complemented by a frost point hygrometer "SnowWhite" and a particle backscatter detector "COBALD" (Compact Optical Backscatter Aerosol Detector). Extensive trajectory calculations based on regional weather model COSMO forecasts are performed for flight planning and COSMO analyses are used as basis for comprehensive microphysical box modeling (with grid scale 2 km and 7 km, respectively). Here we present the results of matching a cirrus cloud to within 2–15 km, realized on 8 June 2010 over Payerne, Switzerland, and a location 120 km downstream close to Zurich. A thick cirrus was detected over both measurement sites. We show that in order to quantitatively reproduce the measured particle backscatter ratios, the small-scale temperature fluctuations not resolved by COSMO must be superimposed on the trajectories. The stochastic nature of the fluctuations is captured by ensemble calculations. Possibilities for further improvements in the agreement with the measured backscatter data are investigated by assuming a very slow mass accommodation of water on ice, the presence of heterogeneous ice nuclei, or a wide span of (spheroidal) particle shapes. However, the resulting improvements from microphysical refinements are moderate and comparable in magnitude with changes caused by assuming different regimes of temperature fluctuations for clear sky or cloudy sky conditions, highlighting the importance of a proper treatment of subscale fluctuations. The model yields good agreement with the measured backscatter over both sites and reproduces the measured saturation ratios with respect to ice over Payerne. Conversely, the 30% in-cloud supersaturation measured in a massive, 4-km thick cloud layer over Zurich cannot be reproduced, irrespective of the choice of meteorological or microphysical model parameters. The measured supersaturation can only be explained by either resorting to an unknown physical process, which prevents the ice particles from consuming the excess humidity, or – much more likely – by a measurement error, such as a contamination of the sensor housing of the SnowWhite hygrometer by a precipitation drop from a mixed phase cloud just below the cirrus layer or from some very slight rain in the boundary layer. This uncertainty calls for in-flight checks or calibrations of hygrometers under the extreme humidity conditions in the upper troposphere.

Description: An explicit study of aerosol mass conversion and its parameterization in warm rain formation of cumulus clouds Atmospheric Chemistry and Physics Discussions, 13, 25481-25536, 2013 Author(s): J. Sun, J. Fen, and R. K. Ungar The life time of atmospheric aerosols is highly affected by in-cloud scavenging processes. Aerosol mass conversion from aerosols embedded in cloud droplets into aerosols embedded in raindrops is a pivotal pathway for wet removal of aerosols in clouds. The aerosol mass conversion rate in the bulk microphysics parameterizations is always assumed to be linearly related to the precipitation production rate, which includes the cloud water autoconversion rate and the cloud water accretion rate. The ratio of the aerosol mass concentration conversion rate to the cloud aerosol mass concentration has typically been considered to be the same as the ratio of the precipitation production rate to the cloud droplet mass concentration. However, the mass of an aerosol embedded in a cloud droplet is not linearly proportional to the mass of the cloud droplet. A simple linear relationship cannot be drawn between the precipitation production rate and the aerosol mass concentration conversion rate. In this paper, we studied the evolution of aerosol mass concentration conversion rates in a warm rain formation process with a 1.5-dimensional non-hydrostatic convective cloud and aerosol interaction model in the bin microphysics. We found that the ratio of the aerosol mass conversion rate to the cloud aerosol mass concentration can be statistically expressed by the ratio of the precipitation production rate to the cloud droplet mass concentration with an exponential function. We further gave some regression equations to determine aerosol conversions in the warm rain formation under different threshold radii of raindrops and different aerosol size distributions.

Description: Global and regional impacts of HONO on the chemical composition of clouds and aerosols Atmospheric Chemistry and Physics Discussions, 13, 23599-23638, 2013 Author(s): Y. F. Elshorban, P. J. Crutzen, B. Steil, A. Pozzer, H. Tost, and J. Lelieveld Nitrous acid (HONO) photolysis can significantly increase HO x (OH+HO 2 ) radical formation, enhancing organic and inorganic oxidation products in polluted regions, especially during winter. It has been reported that chemistry-transport models underestimate sulphate concentrations, mostly during winter. Here we show that HONO can significantly enhance aerosol sulphate (S(VI)), mainly due to the increased formation of H 2 SO 4 . Even though in-cloud aqueous phase oxidation of dissolved SO 2 (S(IV)) is the main source of S(VI), it appears that HONO related enhancement of H 2 O 2 does not significantly affect sulphate because of the predominantly S(IV) limited conditions, except over eastern Asia. Nitrate is also increased via enhanced gaseous HNO 3 formation and N 2 O 5 hydrolysis on aerosol particles. Ammonium nitrate is enhanced in ammonia-rich regions but not under ammonia-limited conditions. Furthermore, particle number concentrations are also higher, accompanied by the transfer from hydrophobic to hydrophilic aerosol modes. This implies a significant impact on the particle lifetime and cloud nucleating properties. The HONO induced enhancements of all species studied are relatively strong in winter though negligible in summer. Simulating realistic HONO levels is found to improve the model-measurement agreement of sulphate aerosols, most apparent over the US. Our results underscore the importance of HONO for the atmospheric oxidizing capacity and the central role of cloud chemical processing in aerosol formation.

Description: Injection heights of springtime biomass burning plumes over the Peninsular Southeast Asia and their impacts on pollutant long-range transport Atmospheric Chemistry and Physics Discussions, 13, 23781-23816, 2013 Author(s): Y. Jian and T.-M. Fu We analyzed observations from the Multi-angle Imaging SpectroRadiometer (MISR) to determine the injection heights of biomass burning smoke plumes over the Peninsular Southeast Asia (PSEA) in spring, with the goal of evaluating the impacts on pollutant long-range transport. We retrieved the heights of twenty-two thousand MISR smoke pixels from 607 smoke plumes over the PSEA during February to April of the years 2001–2010. Forty-five percent of the analyzed smoke pixels were above the local mean boundary layer (1 km) at MISR overpass time (10:30 a.m. local time). We used the GEOS-Chem model to simulate the transport of PSEA biomass burning pollutants in March 2001. We found that the direct injection of 40% of the PSEA biomass burning emissions had little impact on the long-range transport of CO to downwind regions, compared to a control simulation where all biomass burning emissions were released in the boundary layer. This was because CO at the surface over the PSEA was efficiently lifted into the free troposphere by deep convection associated with synoptic-scale weather systems. For pollutants with lifetimes shorter than the synoptic timescale, such as black carbon aerosol (BC), their long-range transport was much more sensitive to the initial plume injection height. The direct injection of NO x from PSEA biomass burning into the free troposphere drove increased formation and transport of PAN, which in turn led to significant increases of ozone over downwind southern China and northwestern Pacific. The Pacific subtropical high transported PSEA biomass burning pollutants to the marine boundary layer over the tropical northwestern Pacific. We compared our model results to aircraft measurements over the northwestern Pacific during the TRACE-P campaign (March 2001). The direct injection of 40% of the PSEA biomass burning pollutants in the free troposphere in the model led to a more pronounced BC peak at 3 km over the northwestern Pacific, which was in better agreement with the aircraft observations compared to the control simulation. Our analyses highlighted the point that the injection heights of smoke plumes pose large uncertainty to the interpretation of BC measurements downwind of biomass burning regions.

Description: Influence of surface morphology on the immersion mode ice nucleation efficiency of hematite particles Atmospheric Chemistry and Physics Discussions, 13, 23757-23780, 2013 Author(s): N. Hiranuma, N. Hoffmann, A. Kiselev, A. Dreyer, K. Zhang, G. Kulkarni, T. Koop, and O. Möhler In this paper, the effect of the morphological modification of aerosol particles with respect to heterogeneous ice nucleation is comprehensively investigated for laboratory-generated hematite particles as a model substrate for atmospheric dust particles. The surface area-scaled ice nucleation efficiencies of monodisperse cubic hematite particles and milled hematite particles were measured with a series of expansion cooling experiments using the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) cloud simulation chamber. Complementary off-line characterization of physico-chemical properties of both hematite subsets were also carried out with scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, dynamic light scattering (DLS), and an electro-kinetic particle charge detector to further constrain droplet-freezing measurements of hematite particles. Additionally, an empirical parameterization derived from our laboratory measurements was implemented in the single-column version of the Community Atmospheric Model version 5 (CAM5) to investigate the model sensitivity in simulated ice crystal number concentration on different ice nucleation efficiencies. From an experimental perspective, our results show that the immersion mode ice nucleation efficiency of milled hematite particles is almost an order of magnitude higher at −35.2 °C 〈 T 〈 −33.5 °C than that of the cubic hematite particles, indicating a substantial effect of morphological irregularities on immersion mode freezing. Our modeling results similarly show that the increased droplet-freezing rates of milled hematite particles lead to about one order magnitude higher ice crystal number in the upper troposphere than cubic hematite particles. Overall, our results suggest that the surface irregularities and associated active sites lead to greater ice activation through droplet-freezing.

Description: Study of the unknown HONO daytime source at an European suburban site during the MEGAPOLI summer and winter field campaigns Atmospheric Chemistry and Physics Discussions, 13, 23639-23690, 2013 Author(s): V. Michoud, A. Colomb, A. Borbon, K. Miet, M. Beekmann, M. Camredon, B. Aumont, S. Perrier, P. Zapf, G. Siour, W. Ait-Helal, C. Afif, A. Kukui, M. Furger, J. C. Dupont, M. Haeffelin, and J. F. Doussin Nitrous acid measurements were carried out during the MEGAPOLI summer and winter field campaigns at SIRTA observatory in Paris surroundings. Highly variable HONO levels were observed during the campaigns, ranging from 10 ppt to 500 ppt in summer and from 10 ppt to 1.7 ppb in winter. Significant HONO mixing ratios have also been measured during daytime hours, comprised between some tenth of ppt and 200 ppt for the summer campaign and between few ppt and 1 ppb for the winter campaign. Ancillary measurements, such as NO x , O 3 , photolysis frequencies, meteorological parameters (pressure, temperature, relative humidity, wind speed and wind direction), black carbon concentration, total aerosol surface area, boundary layer height and soil moisture, were conducted during both campaigns. In addition, for the summer period, OH radical measurements were made with a CIMS (Chemical Ionisation Mass Spectrometer). This large dataset has been used to investigate the HONO budget in a suburban environment. To do so, calculations of HONO concentrations using PhotoStationary State (PSS) approach have been performed, for daytime hours. The comparison of these calculations with measured HONO concentrations revealed an underestimation of the calculations making evident a missing source term for both campaigns. This unknown HONO source exhibits a bell shaped like average diurnal profile with a maximum around noon of approximately 0.7 ppb h −1 and 0.25 ppb h −1 , during summer and winter respectively. This source is the main HONO source during daytime hours for both campaigns. In both cases, this source shows a slight positive correlation with J (NO 2 ) and the product between J (NO 2 ) and soil moisture. This original approach had, thus, indicated that this missing source is photolytic and might be heterogeneous occurring on ground surface and involving water content available at the ground.

Description: Relationship between Amazon biomass burning aerosols and rainfall over La Plata Basin Atmospheric Chemistry and Physics Discussions, 13, 23995-24021, 2013 Author(s): G. Camponogara, M. A. F. Silva Dias, and G. G. Carrió High aerosol loads are discharged into the atmosphere by biomass burning in Amazon and Central Brazil during the dry season. These particles can interact with clouds as cloud condensation nuclei (CCN) changing cloud microphysics and radiative properties and, thereby, affecting the radiative budget of the region. Furthermore, the biomass burning aerosols can be transported by the low level jet (LLJ) to La Plata Basin where many mesoscale convective systems (MCS) are observed during spring and summer. This work proposes to investigate whether the aerosols from biomass burning may affect the MCS in terms of rainfall over La Plata Basin during spring. Since the aerosol effect is very difficult to isolate because convective clouds are very sensitive to small environment disturbances, detailed analyses using different techniques are used. The binplot, 2D histograms and combined empirical orthogonal function (EOF) methods are used to separate certain environment conditions with the possible effects of aerosol loading. Reanalysis 2, TRMM-3B42 and AERONET data are used from 1999 up to 2012 during September-December. The results show that there are two patterns associated to rainfall-aerosol interaction in La Plata Basin: one in which the dynamic conditions are more important than aerosols to generate rain; and a second one where the aerosol particles have a role in rain formation, acting mainly to suppress rainfall over La Plata Basin.

Description: Size-resolved aerosol composition and link to hygroscopicity at a forested site in Colorado Atmospheric Chemistry and Physics Discussions, 13, 23817-23843, 2013 Author(s): E. J. T. Levin, A. J. Prenni, B. Palm, D. Day, P. Campuzano-Jost, P. M. Winkler, S. M. Kreidenweis, P. J. DeMott, J. Jimenez, and J. N. Smith Aerosol hygroscopicity describes the ability of a particle to take up water and form a cloud droplet. Modeling studies have shown sensitivity of precipitation-producing cloud systems to the availability of aerosol particles capable of serving as cloud condensation nuclei (CCN), and hygroscopicity is a key parameter controlling the number of available CCN. Continental aerosol is typically assumed to have a representative hygroscopicity parameter, κ, of 0.3; however, in remote locations this value can be lower due to relatively large mass fractions of organic components. To further our understanding of aerosol properties in remote areas, we measured size-resolved aerosol chemical composition and hygroscopicity in a forested, mountainous site in Colorado during the six-week BEACHON-RoMBAS campaign. This campaign followed a year-long measurement period at this site, and results from the intensive campaign shed light on the previously reported seasonal cycle in aerosol hygroscopicity. New particle formation events were observed routinely at this site and nucleation mode composition measurements indicated that the newly formed particles were predominantly organic. These events likely contribute to the dominance of organic species at smaller sizes, where aerosol organic mass fractions of non-refractory components were between 70–90%. Corresponding aerosol hygroscopicity was observed to range from κ = 0.15–0.22, with hygroscopicity increasing with particle size. Aerosol chemical composition measured by an Aerosol Mass Spectrometer and calculated from hygroscopicity measurements agreed very well during the intensive study with an assumed value of κ org = 0.13 resulting in the best agreement.

Description: Atmospheric parameters in a subtropical cloud regime transition derived by AIRS+MODIS – observed statistical variability compared to ERA-Interim Atmospheric Chemistry and Physics Discussions, 13, 24051-24085, 2013 Author(s): M. M. Schreier, B. H. Kahn, K. Sušelj, J. Karlsson, S. C. Ou, Q. Yue, and S. L. Nasiri Cloud occurrence, microphysical and optical properties and atmospheric profiles within a subtropical cloud regime transition in the northeastern Pacific Ocean are obtained from a synergistic combination of the Atmospheric Infrared Sounder (AIRS) and the MODerate resolution Imaging Spectroradiometer (MODIS). The observed cloud parameters and atmospheric thermodynamic profile retrievals are binned by cloud type and analyzed based on their probability density functions (PDFs). Comparison of the PDFs to data from the European Center for Medium Range Weather Forecasting Re-analysis (ERA-Interim) shows a strong difference in the occurrence of the different cloud types compared to clear sky. An increasing non-Gaussian behavior is observed in cloud optical thickness (τ c ), effective radius ( r e ) and cloud top temperature ( T c ) distributions from Stratocumulus to Trade Cumulus, while decreasing values of lower tropospheric stability are seen. However, variations in the mean, width and shape of the distributions are found. The AIRS potential temperature (θ) and water vapor ( q ) profiles in the presence of varying marine boundary layer (MBL) cloud types show overall similarities to the ERA-Interim in the mean profiles, but differences arise in the higher moments at some altitudes. The differences between the PDFs from AIRS+MODIS and ERA-Interim make it possible to pinpoint systematic errors in both systems and helps to understand joint PDFs of cloud properties and coincident thermodynamic profiles from satellite observations.

Description: Establishing the contribution of lawn mowing to atmospheric aerosol levels in American suburbs Atmospheric Chemistry and Physics Discussions, 13, 24435-24480, 2013 Author(s): R. M. Harvey, J. Zahardis, and G. A. Petrucci Green leaf volatiles (GLVs) are a class of wound-induced volatile organic compounds emitted by several plant species. Turfgrasses emit a complex profile of GLVs upon mowing, as evidenced by the "freshly cut grass" smell, some of which are readily oxidized in the atmosphere to contribute to secondary organic aerosol (SOA). The contribution of lawn mowing-induced SOA production may be especially impactful at the urban/suburban interface, where urban hubs provide a source of anthropogenic oxidants and SOA while suburban neighborhoods have the potential to emit large quantities of reactive, mow-induced GLVs. This interface provides a unique opportunity to study aerosol formation in a multi-component system and at a regionally relevant scale. Freshly cut grass was collected from a study site in Essex Junction, Vermont and was placed inside a 775 L Teflon experimental chamber. Thermal desorption gas chromatography mass spectrometry (TD-GC/MS) was used to characterize the emitted GLV profile. Ozone was introduced to the experimental chamber and TD-GC/MS was used to monitor the consumption of these GLVs and the subsequent evolution of gas phase products while a scanning mobility particle sizer was used to continuously measure aerosol size distributions and mass loadings as a result of grass clipping ozonolysis. Freshly cut grass found to emit a complex mixture of GLVs, dominated by cis -3-hexenyl acetate and cis -3-hexenol, which were released at an initial rate of 1.8 (±0.5) μg and 0.07 (±0.03) μg per square meter of lawn mowed with each mowing. Chamber studies using pure standards of cis -3-hexenyl acetate (CHA) and cis -3-hexenol (HXL) were found to have aerosol yields of 1.2 (±1.1)% and 3.3 (±3.1)%, respectively. Using these aerosol yields and the emission rate of these CHA and HXL by grass, SOA evolution by ozonolysis of grass clippings was predicted. However, the measured SOA mass produced from the ozonolysis of grass clippings exceeded the predicted amount, by upwards of ~ 150%. The ozonolysis of a mixture of CHA and HXL representative of environmental mixing ratios also failed to accurately model the SOA mass produced by grass clippings. Aerial photographs and geospatial analysis were used to determine the turfgrass coverage in a suburban neighborhood, which was then used along with measured SOA production as a function of grass mowed to determine that lawn mowing has the potential to contribute 47 μg m −2 SOA to the atmosphere per mowing event by ozonolysis, which cannot be modeled solely by the ozonolysis of CHA, HXL or a representative mixture of the two.

Description: Global distributions and trends of atmospheric ammonia (NH 3 ) from IASI satellite observations Atmospheric Chemistry and Physics Discussions, 13, 24301-24342, 2013 Author(s): M. Van Damme, L. Clarisse, C. L. Heald, D. Hurtmans, Y. Ngadi, C. Clerbaux, A. J. Dolman, J. W. Erisman, and P. F. Coheur Ammonia (NH 3 ) emissions in the atmosphere have strongly increased in the past decades, largely because of the intensive livestock production and use of fertilizers. As a short-lived species, NH 3 is highly variable in the atmosphere and its concentration is generally small, except in and close to local source areas. While ground-based measurements are possible, they are challenging and sparse. Advanced infrared sounders in orbit have recently demonstrated their capability to measure NH 3 , offering a new tool to refine global and regional budgets. In this paper we describe an improved retrieval scheme of NH 3 total columns from the measurements of the Infrared Atmospheric Sounding Interferometer (IASI). It exploits the hyperspectral character of this instrument by using an extended spectral range (800–1200 cm −1 ) where NH 3 is optically active. This scheme consists of the calculation of a dimensionless spectral index from the IASI level1C radiances, which is subsequently converted to a total NH 3 column using look-up-tables built from forward radiative transfer model simulations. We show how to retrieve the NH 3 total columns from IASI quasi-globally and twice daily, above both land and sea, without large computational resources and with an improved detection limit. The retrieval also provides error characterization on the retrieved columns. Five years of IASI measurements (1 November 2007 to 31 October 2012) have been processed to acquire the first global and multiple-year dataset of NH 3 total columns, which are evaluated and compared to similar products from other retrieval methods. Spatial distributions from the five years dataset are provided and analyzed at global and regional scales. We show in particular the ability of this method to identify smaller emission sources than those reported previously, as well as transport patterns above sea. The five year time series is further examined in terms of seasonality and inter-annual variability (in particular as a function of fire activity) separately for the Northern and Southern Hemispheres.

Description: Aircraft measurements of polar organic tracer compounds in tropospheric particles (PM 10 ) over Central China Atmospheric Chemistry and Physics Discussions, 13, 24481-24516, 2013 Author(s): P. Q. Fu, K. Kawamura, Y. F. Cheng, S. Hatakeyama, A. Takami, H. Li, and W. Wang Atmospheric aerosol samples were collected by aircraft at low to middle altitudes (0.8–3.5 km a.g.l.) over Central East to West China during summer 2003 and spring 2004. The samples were analyzed for polar organic compounds using a technique of solvent extraction/BSTFA derivatization/gas chromatography-mass spectrometry. Biogenic secondary organic aerosol (SOA) tracers from the oxidation of isoprene were found to be more abundant in summer (3.3–138 ng m −3 , mean 39 ng m −3 ) than in spring (3.2–42 ng m −3 , 15 ng m −3 ), while α/β-pinene and β-caryophyllene SOA tracers showed similar abundance between these two seasons. A strong positive correlation ( R 2 =0.83) between levoglucosan and β-caryophyllinic acid was found in the spring samples versus a weak correlation ( R 2 =0.17) in the summer samples, implying substantial contributions from biomass burning to the β-caryophyllinic acid production in spring. Two organic nitrogen species (oxamic acid and carbamide) were detected in the aircraft aerosol samples and their concentrations were comparable to those of biogenic SOA tracers. Most of the POA and SOA tracers were less abundant at higher altitudes, suggesting they are of ground surface origin, either being directly emitted from anthropogenic/natural sources on the ground surface, or rapidly formed through photooxidation of their precursors emitted from the ground surface and then diluted during uplifting into the troposphere. This study demonstrates that primary biological aerosols, biogenic SOA, and organic nitrogen species are important components of organic aerosols in the troposphere over Central China.

Description: Black carbon emissions from in-use ships: a California regional assessment Atmospheric Chemistry and Physics Discussions, 13, 24675-24712, 2013 Author(s): G. M. Buffaloe, D. A. Lack, E. J. Williams, D. Coffman, K. L. Hayden, B. M. Lerner, S-M. Li, I. Nuaaman, P. Massoli, T. B. Onasch, P. K. Quinn, and C. D. Cappa Black carbon (BC) mass emission factors (EF BC ; g-BC (kg-fuel) −1 ) from a variety of ocean going vessels have been determined from measurements of BC and carbon dioxide (CO 2 ) concentrations in ship plumes intercepted by the R/V Atlantis during the 2010 California Nexus (CalNex) campaign. The ships encountered were all operating within 24 nautical miles of the California coast and were utilizing relatively low sulphur fuels. Black carbon concentrations within the plumes, from which EF BC values are determined, were measured using four independent instruments: a photoacoustic spectrometer and a particle soot absorption photometer, which measure light absorption, and a single particle soot photometer and soot particle aerosol mass spectrometer, which measure the mass concentration of refractory BC directly. The measured EF BC have been divided into vessel type categories and engine type categories, from which averages have been determined. The geometric average EF BC , determined from over 71 vessels and 135 plumes encountered, was 0.31 g-BC (kg-fuel) −1 . The most frequent engine type encountered was the slow speed diesel (SSD), and the most frequent SSD vessel type was the cargo ship sub-category. Average and median EF BC values from the SSD category are compared with previous observations from the Texas Air Quality Study (TexAQS) in 2006, during which the ships encountered were predominately operating on high sulphur fuels. There is a statistically significant difference between the EF BC values from CalNex and TexAQS for SSD vessels and for the cargo and tanker ship types within this engine category. The CalNex EF BC values are lower than those from TexAQS, suggesting that operation on lower sulphur fuels is associated with smaller EF BC values.

Description: A case study into the measurement of ship emissions from plume intercepts of the NOAA Ship Miller Freeman Atmospheric Chemistry and Physics Discussions, 13, 24635-24674, 2013 Author(s): C. D. Cappa, E. J. Williams, D. A. Lack, G. M. Buffaloe, D. Coffman, K. L. Hayden, S. C. Herndon, B. M. Lerner, S-M. Li, P. Massoli, R. McLaren, I. Nuaaman, T. B. Onasch, and P. K. Quinn Emissions factors (EFs) for gas and sub-micron particle-phase species were measured in intercepted plumes as a function of vessel speed from an underway research vessel, the NOAA Ship Miller Freeman , operating a medium-speed diesel engine on low-sulfur marine gas oil. For many of the particle-phase species, EFs were determined using multiple measurement methodologies, allowing for an assessment of how well EFs from different techniques agree. The total sub-micron PM (PM 1 ) was dominated by particulate black carbon (BC) and particulate organic matter (POM), with an average POM / BC ratio of 1.3. Consideration of the POM / BC ratios observed here with literature studies suggests that laboratory and in-stack measurement methods may over-estimate primary POM EFs relative to those observed in emitted plumes. Comparison of four different methods for black carbon measurement indicates that careful attention must be paid to instrument limitations and biases when assessing EF BC . Particulate sulfate (SO 4 2− ) EFs were extremely small and the particles emitted by Miller Freeman were inefficient as cloud condensation nuclei (CCN), even at high super saturations, consistent with the use of very low sulfur fuel and the overall small emitted particle sizes. All measurement methodologies consistently demonstrate that the measured EFs (fuel mass basis) for PM 1 mass, BC and POM decreased as the ship slowed. Particle number EFs were approximately constant across the speed change, with a shift towards smaller particles being emitted at slower speeds. Emissions factors for gas-phase CO and formaldehyde (HCHO) both increased as the vessel slowed, while EFs for NO x decreased and SO 2 EFs were approximately constant.

Description: Heterogeneous reaction of N 2 O 5 with illite and Arizona Test Dust particles Atmospheric Chemistry and Physics Discussions, 13, 24855-24884, 2013 Author(s): M. J. Tang, G. Schuster, and J. N. Crowley The heterogeneous reaction of N 2 O 5 with airborne illite and Arizona Test Dust particles was investigated at room temperature and at different relative humidities using an atmospheric pressure aerosol flow tube. N 2 O 5 at concentrations in the range 8 to 24×10 12 molecule cm −3 was monitored using thermal-dissociation cavity ring-down spectroscopy at 662 nm. At zero relative humidity a large uptake coefficient of N 2 O 5 to illite was obtained, γ(N 2 O 5 ) = 0.09, which decreased to 0.04 as relative humidity was increased to 67%. In contrast, the uptake coefficient derived for ATD is much lower (~ 0.006) and, within experimental uncertainty, independent of relative humidity (0–67%). Potential explanations are given for the significant differences between the uptake behaviour for ATD and illite and the results are compared with uptake coefficients for N 2 O 5 on other mineral surfaces.

Description: A case study of sea breeze blocking regulated by sea surface temperature along the English south coast Atmospheric Chemistry and Physics Discussions, 13, 24785-24807, 2013 Author(s): J. K. Sweeney, J. M. Chagnon, and S. L. Gray The sensitivity of sea breeze structure to sea surface temperature (SST) and coastal orography is investigated in convection-permitting Met Office Unified Model simulations of a case study along the south coast of England. Changes in SST of 1 K are shown to significantly modify the structure of the sea breeze. On the day of the case study the sea breeze was partially blocked by coastal orography, particularly within Lyme Bay. The extent to which the flow is blocked depends strongly on the static stability of the marine boundary layer. In experiments with colder SST, the marine boundary layer is more stable, and the degree of blocking is more pronounced. The implications of prescribing fixed SST from climatology in numerical weather prediction model forecasts of the sea breeze are discussed.

Description: Using a WRF simulation to examine regions where convection impacts the Asian summer monsoon anticyclone Atmospheric Chemistry and Physics Discussions, 13, 24809-24853, 2013 Author(s): N. K. Heath and H. E. Fuelberg The Asian summer monsoon is a prominent feature of the global circulation that is associated with an upper-level anticyclone (ULAC) that stands out vividly in satellite observations of trace gases. The ULAC also is an important region of troposphere-to-stratosphere transport. We ran the Weather Research and Forecasting (WRF) model at convective-permitting scales (4 km grid spacing) between 10–20 August 2012 to understand the role of convection in transporting boundary layer air into the upper-level anticyclone. Such high-resolution modeling of the Asian ULAC previously has not been documented in the literature. Comparison of our WRF simulation with reanalysis and satellite observations showed that WRF simulated the atmosphere sufficiently well to be used to study convective transport into the ULAC. A back-trajectory analysis based on hourly WRF output showed that 〉 90% of convectively influenced parcels reaching the ULAC came from the Tibetan Plateau (TP) and the southern slope (SS) of the Himalayas. A distinct diurnal cycle is seen in the convective trajectories, with their greatest impact occurring between 1600–2300 local solar time. This finding highlights the role of "everyday" diurnal convection in transporting boundary layer air into the ULAC. WRF output at 15 min intervals was produced for 16 August to examine the convection in greater detail. This high-temporal output revealed that the weakest convection in the study area occurred over the TP. However, because the TP is at 3000–5000 m a.m.s.l., its convection does not have to be as strong to reach the ULAC as in lower altitude regions. In addition, because the TP's elevated heat source is a major cause of the ULAC, we propose that convection over the TP and the neighboring SS is ideally situated geographically to impact the ULAC. The vertical mass flux of water vapor into the ULAC also was calculated. Results show that the TP and SS regions dominate other Asian regions in transporting moisture vertically into the ULAC. Because convection reaching the ULAC is more widespread over the TP than nearby, we propose that the abundant convection partially explains the TP's dominant water vapor fluxes. In addition, greater outgoing longwave radiation reaches the upper levels of the TP due to its elevated terrain. This creates a warmer ambient upper level environment, allowing parcels with greater saturation mixing ratios to enter the ULAC. Lakes in the Tibetan Plateau are shown to provide favorable conditions for deep convection during the night.

Description: An assessment of the performance of the Monitor for AeRosols and GAses in ambient air (MARGA): a semi-continuous method for soluble compounds Atmospheric Chemistry and Physics Discussions, 13, 25067-25124, 2013 Author(s): I. C. Rumsey, K. A. Cowen, J. T. Walker, T. J. Kelly, E. A. Hanft, K. Mishoe, C. Rogers, R. Proost, G. M. Beachley, G. Lear, T. Frelink, and R. P. Otjes Ambient air monitoring as part of the US Environmental Protection Agency's (US EPA's) Clean Air Status and Trends Network (CASTNet) currently uses filter packs to measure weekly integrated concentrations. The US EPA is interested in supplementing CASTNet with semi-continuous monitoring systems at select sites to characterize atmospheric chemistry and deposition of nitrogen and sulfur compounds at higher time resolution than the filter pack. The Monitor for AeRosols and GAses in ambient air (MARGA) measures water-soluble gases and aerosols at hourly temporal resolution. The performance of the MARGA was assessed under the US EPA Environmental Technology Verification (ETV) program. The assessment was conducted in Research Triangle Park, North Carolina from 8 September–8 October 2010 and focused on gaseous SO 2 , HNO 3 and NH 3 and aerosol SO 4 − , NO 3 − and NH 4 + . Precision of the MARGA was evaluated by calculating the median absolute relative percent difference (MARPD) between paired hourly results from duplicate MARGA units (MUs), with a performance goal of

Description: Sensitivity of simulated climate to latitudinal distribution of solar insolation reduction in SRM geoengineering methods Atmospheric Chemistry and Physics Discussions, 13, 25387-25415, 2013 Author(s): A. Modak and G. Bala Solar radiation management (SRM) geoengineering has been proposed as a potential option to counteract climate change. We perform a set of idealized geoengineering simulations to understand the global hydrological implications of varying the latitudinal distribution of solar insolation reduction in SRM methods. We find that for a fixed total mass of sulfate aerosols (12.6 Mt of SO 4 ), relative to a uniform distribution which mitigates changes in global mean temperature, global mean radiative forcing is larger when aerosol concentration is maximum at the poles leading to a warmer global mean climate and consequently an intensified hydrological cycle. Opposite changes are simulated when aerosol concentration is maximized in the tropics. We obtain a range of 1 K in global mean temperature and 3% in precipitation changes by varying the distribution pattern: this range is about 50% of the climate change from a doubling of CO 2 . Hence, our study demonstrates that a range of global mean climate states, determined by the global mean radiative forcing, are possible for a fixed total amount of aerosols but with differing latitudinal distribution, highlighting the need for a careful evaluation of SRM proposals.

Description: Source apportionment of PM 10 in a North-Western Europe regional urban background site (Lens, France) using Positive Matrix Factorization and including primary biogenic emissions Atmospheric Chemistry and Physics Discussions, 13, 25325-25385, 2013 Author(s): A. Waked, O. Favez, L. Y. Alleman, C. Piot, J.-E. Petit, T. Delaunay, E. Verlinden, B. Golly, J.-L. Besombes, J.-L. Jaffrezo, and E. Leoz-Garziandia In this work, the source of ambient particulate matter (PM 10 ) collected over a one year period at an urban background site in Lens (France) were determined and investigated using a~Positive Matrix Factorization receptor model (US EPA PMF v3.0). In addition, a Potential Source Contribution Function (PSCF) was performed by means of the Hysplit v4.9 model to assess prevailing geographical origins of the identified sources. A selective iteration process was followed for the qualification of the more robust and meaningful PMF solution. Components measured and used in the PMF include inorganic and organic species: soluble ionic species, trace elements, elemental carbon (EC), sugars alcohols, sugar anhydride, and organic carbon (OC). The mean PM 10 concentration measured from March 2011 to March 2012 was about 21 μg m −3 with typically OM, nitrate and sulfate contributing to most of the mass and accounting respectively for 5.8, 4.5 and 2.3 μg m −3 on a yearly basis. Accordingly, PMF outputs showed that the main emission sources were (in a decreasing order of contribution): secondary inorganic aerosols (28% of the total PM 10 mass), aged marine emissions (19%), with probably predominant contribution of shipping activities, biomass burning (13%), mineral dust (13%), primary biogenic emissions (9%), fresh sea salts (8%), primary traffic emissions (6%) and heavy oil combustion (4%). Significant temporal variations were observed for most of the identified sources. In particular, biomass burning emissions were negligible in summer but responsible for about 25% of total PM 10 and 50% of total OC at wintertime. Conversely, primary biogenic emissions were found to be negligible in winter but to represent about 20% of total PM 10 and 40% of total OC in summer. The latter result calls for more investigations of primary biogenic aerosols using source apportionment studies, which quite usually disregards this type of sources. This study furthermore underlines the major influence of secondary processes during daily threshold exceedances. Finally, apparent discrepancies that could be generally observed between filter-based studies (such as the present one) and Aerosol Mass Spectrometer-based PMF analyses (organic fractions) are also discussed here.

Description: Wind extraction potential from 4D-Var assimilation of O 3 , N 2 O, and H 2 O using a global shallow water model Atmospheric Chemistry and Physics Discussions, 13, 25291-25323, 2013 Author(s): D. R. Allen, K. W. Hoppel, and D. D. Kuhl The wind extraction due to assimilation of trace gas (tracer) data is examined using a 4D-Var data assimilation system based on the shallow water equations coupled to the tracer continuity equation. The procedure is outlined as follows. First, a Nature Run is created, simulating middle stratospheric winter conditions. Second, ozone (O 3 ), nitrous oxide (N 2 O), and water vapor (H 2 O) (treated in this study as passive tracers) are initialized using Microwave Limb Sounder (MLS) mixing ratios at 850 K potential temperature and advected by the Nature Run winds. Third, the initial dynamical conditions are perturbed by using a 6 h offset. Fourth, observations based on the simulated tracer data are then assimilated with a 4D-Var system in which the tracer and winds are coupled via the adjoint of the tracer continuity equation. Finally, the wind extraction potential (WEP) is calculated as the reduction of the Root Mean Square (RMS) vector wind error due to tracer assimilation relative to the total possible reduction from the initial perturbed conditions. For a single 6 h assimilation cycle of "perfect" tracer (unbiased and no imposed random errors), WEP values are 70% for O 3 , 49% for N 2 O and 16% for H 2 O. O 3 and N 2 O provide more wind information than H 2 O due to stronger background gradients relative to the tracer precisions. 10 day multi-cycle simulations with "perfect" tracer result in WEP of 98% for O 3 , 97% for N 2 O, and 90% for H 2 O. There is therefore sufficient information in these fields to nearly completely specify the dynamics, even without assimilation of dynamical information. For assimilation of tracer observations with realistic random noise (based on MLS precision at 10 hPa), the WEP after 10 days decreases to 57% for O 3 , 42% for N 2 O, and 28% for H 2 O. The root-mean-square (RMS) wind errors level out at ~ 1–2 m s −1 for these runs, suggesting a limit to which realistic tracers could constrain the winds, given complete global coverage. With higher observation noise levels, the WEP values decrease further, with negative WEP occurring in cases of very large errors for H 2 O, indicating that assimilation of very noisy observations may worsen the wind fields.

Description: The simulations of sulfuric acid concentration and new particle formation in an urban atmosphere in China Atmospheric Chemistry and Physics Discussions, 13, 14977-15005, 2013 Author(s): Z. B. Wang, M. Hu, D. Mogensen, D. L. Yue, J. Zheng, R. Y. Zhang, Y. Liu, B. Yuan, X. Li, M. Shao, L. Zhou, Z. J. Wu, A. Wiedensohler, and M. Boy Simulations of sulfuric acid concentration and new particle formation are performed by using the zero-dimensional version of the model MALTE (Model to predict new Aerosol formation in the Lower TropospherE) and measurements from the Campaign of Air Quality Research in Beijing and Surrounding areas (CAREBeijing) in 2008. Chemical reactions from the Master Chemical Mechanism Version 3.2 (MCM v3.2) are used in the model. High correlation (slope = 0.72, R = 0.74) between the modelled and observed sulfuric acid concentrations is found during daytime (06:00–18:00). The aerosol dynamics are simulated by the University of Helsinki Multicomponent Aerosol (UHMA) model including several nucleation mechanisms. The results indicate that the model is able to predict the on- and offset of new particle formation in an urban atmosphere in China. In addition, the number concentrations of newly formed particles in kinetic-type nucleation including homogenous homomolecular ( J=K [H 2 SO 4 ] 2 ) and homogenous heteromolecular nucleation involving organic vapours ( J=K het [H 2 SO 4 ][Org]) are in satisfactory agreement with the observations. However, the specific organic compounds possibly participate in the nucleation process should be investigated in further studies.

Description: Reductions in aircraft particulate emissions due to the use of Fischer–Tropsch fuels Atmospheric Chemistry and Physics Discussions, 13, 15105-15139, 2013 Author(s): A. J. Beyersdorf, M. T. Timko, L. D. Ziemba, D. Bulzan, E. Corporan, S. C. Herndon, R. Howard, R. Miake-Lye, K. L. Thornhill, E. Winstead, C. Wey, Z. Yu, and B. E. Anderson The use of alternative fuels for aviation is likely to increase due to concerns over fuel security, price stability and the sustainability of fuel sources. Concurrent reductions in particulate emissions from these alternative fuels are expected because of changes in fuel composition including reduced sulfur and aromatic content. The NASA Alternative Aviation Fuel Experiment (AAFEX) was conducted in January–February 2009 to investigate the effects of synthetic fuels on gas-phase and particulate emissions. Standard petroleum JP-8 fuel, pure synthetic fuels produced from natural gas and coal feedstocks using the Fischer–Tropsch (FT) process, and 50% blends of both fuels were tested in the CFM-56 engines on a DC-8 aircraft. To examine plume chemistry and particle evolution with time, samples were drawn from inlet probes positioned 1, 30, and 145 m downstream of the aircraft engines. No significant alteration to engine performance was measured when burning the alternative fuels. However, leaks in the aircraft fuel system were detected when operated with the pure FT fuels as a result of the absence of aromatic compounds in the fuel. Dramatic reductions in soot emissions were measured for both the pure FT fuels (reductions of 84% averaged over all powers) and blended fuels (64%) relative to the JP-8 baseline with the largest reductions at idle conditions. The alternative fuels also produced smaller soot (e.g. at 85% power, volume mean diameters were reduced from 78 nm for JP-8 to 51 nm for the FT fuel), which may reduce their ability to act as cloud condensation nuclei (CCN). The reductions in particulate emissions are expected for all alternative fuels with similar reductions in fuel sulfur and aromatic content regardless of the feedstock. As the plume cools downwind of the engine, nucleation-mode aerosols form. For the pure FT fuels, reductions (94% averaged over all powers) in downwind particle number emissions were similar to those measured at the exhaust plane (84%). However, the blended fuels had less of a reduction (reductions of 30–44%) than initially measured (64%). The likely explanation is that the reduced soot emissions in the blended fuel exhaust plume results in promotion of new particle formation microphysics, rather than coating on pre-existing soot particles, which is dominant in the JP-8 exhaust plume. Downwind particle volume emissions were reduced for both the pure (79 and 86% reductions) and blended FT fuels (36 and 46%) due to the large reductions in soot emissions. In addition, the alternative fuels had reduced particulate sulfate production (near-zero for FT fuels) due to decreased fuel sulfur content. To study the formation of volatile aerosols (defined as any aerosol formed as the plume ages) in more detail, tests were performed at varying ambient temperatures (−4 to 20 °C). At idle, particle number and volume emissions were reduced linearly with increasing ambient temperature, with best fit slopes corresponding to −1.2 × 10 6 # (kg fuel) −1 °C −1 for particle number emissions and −9.7 mm 3 (kg fuel) −1 °C −1 for particle volume emissions. The temperature dependence of aerosol formation can have large effects on local air quality surrounding airports in cold regions. Aircraft produced aerosols in these regions will be much larger than levels expected based solely on measurements made directly at the engine exit plane. The majority (90% at idle) of the volatile aerosol mass formed as nucleation-mode aerosols with a smaller fraction as a soot coating. Conversion efficiencies of up to 3.8% were measured for the partitioning of gas-phase precursors (unburned hydrocarbons and SO 2 ) to form volatile aerosols. Highest conversion efficiencies were measured at 45% power.

Description: Undisturbed and disturbed above canopy ponderosa pine emissions: PTR-TOF-MS measurements and MEGAN 2.1 model results Atmospheric Chemistry and Physics Discussions, 13, 15333-15375, 2013 Author(s): L. Kaser, T. Karl, A. Guenther, M. Graus, R. Schnitzhofer, A. Turnipseed, L. Fischer, P. Harley, M. Madronich, D. Gochis, F. N. Keutsch, and A. Hansel We present the first eddy covariance flux measurements of volatile organic compounds (VOCs) using a proton-transfer-reaction time-of-flight mass-spectrometer (PTR-TOF-MS) above a ponderosa pine forest in Colorado, USA. The high mass resolution of the PTR-TOF-MS enabled the identification of chemical sum formulas. During a 30 day measurement period in August and September 2010, 649 different ion mass peaks were detected in the ambient air mass spectrum (including primary ions and mass calibration compounds). Eddy covariance with the vertical wind speed was calculated for all ion mass peaks. On a typical day, 17 ion mass peaks including protonated parent compounds, their fragments and isotopes as well as VOC-H + -water clusters showed a significant flux with daytime average emissions above a reliable flux threshold of 0.1 mg compound m −2 h −1 . These ion mass peaks could be assigned to seven compound classes. The main flux contributions during daytime (10:00–18:00 LT) are attributed to the sum of 2-methyl-3-buten-2-ol (MBO) and isoprene (50%), methanol (12%), the sum of acetic acid and glycolaldehyde (10%) and the sum of monoterpenes (10%). The total MBO + isoprene flux was composed of 10% isoprene and 90% MBO. There was good agreement between the light and temperature dependency of the sum of MBO and isoprene observed for this work and those of earlier studies. The above canopy flux measurements of the sum of MBO and isoprene and the sum of monoterpenes were compared to emissions calculated using the Model of Emissions of Gases and Aerosols from Nature (MEGAN 2.1). The best agreement between MEGAN 2.1 and measurements was reached using emission factors determined from site specific leaf cuvette measurements. While the modelled and measured MBO + isoprene fluxes agree well the emissions of the sum of monoterpenes is underestimated by MEGAN 2.1. This is expected as some factors impacting monoterpene emissions, such as physical damage of needles and branches due to storms, are not included in MEGAN 2.1. After a severe hailstorm event, 22 ion mass peaks (attributed to six compound classes plus some unknown compounds) showed an elevated flux for the two following days. The sum of monoterpene emissions was 4–23 times higher compared to emissions prior to the hailstorm while MBO emissions remained unchanged. If one heavy storm occurs at this site every month we calculate that the monthly monoterpene emissions (in mg compound m −2 ) would be underestimated by 40% if this disturbance source is not considered.

Description: Aerosol variability and atmospheric transport in the Himalayan region from CALIOP 2007–2010 observations Atmospheric Chemistry and Physics Discussions, 13, 15271-15299, 2013 Author(s): S. Bucci, C. Cagnazzo, F. Cairo, L. Di Liberto, and F. Fierli Himalayan Plateau is surrounded by regions with high natural and anthropogenic aerosol emissions that have a strong impact on regional climate. This is particularly critical for the Himalayan glaciers whose equilibrium is also largely influenced by radiative direct and indirect effects induced by aerosol burden. This work focuses on the spatial and vertical distribution of different aerosol types, their seasonal variability and sources. The analysis of the 2007–2010 yr of CALIPSO vertically resolved satellite data allows the identification of spatial patterns of desert dust and carbonaceous particles in different atmospheric layers. Clusters of Lagrangian back-trajectories highlight the transport pathways from source regions during the dusty spring season. The analysis shows a prevalence of dust; at low heights they are distributed mainly north (with a main contribution from the Gobi and Taklamakan deserts) and west of the Tibetan Plateau (originating from the deserts of South-West Asia and advected by the westerlies). Above the Himalayas the dust amount is minor but still not negligible (detectable in around 20% of the measurements), and transport from more distant deserts (Sahara and Arabian Peninsula) is important. Smoke aerosol, produced mainly in North India and East China, is subject to shorter range transport and is indeed observed closer to the sources while there is a limited amount reaching the top of the plateau. Data analysis reveals a clear seasonal variability in the frequencies of occurrence for the main aerosol types; dust is regulated principally by the monsoon dynamics, with maxima of occurrence in spring. The study also highlights relevant interannual differences, showing a larger presence of aerosol in the region during 2007 and 2008 yr.

Description: Quantifying aerosol mixing state with entropy and diversity measures Atmospheric Chemistry and Physics Discussions, 13, 15615-15662, 2013 Author(s): N. Riemer and M. West This paper presents the first quantitative metric for aerosol population mixing state, defined as the distribution of per-particle chemical species composition. This new metric, the mixing state index χ, is an affine ratio of the average per-particle species diversity D α and the bulk population species diversity D γ , both of which are based on information-theoretic entropy measures. The mixing state index χ enables the first rigorous definition of the spectrum of mixing states from so-called external mixture to internal mixture, which is significant for aerosol climate impacts, including aerosol optical properties and cloud condensation nuclei activity. We illustrate the usefulness of this new mixing state framework with model results from the stochastic particle-resolved model PartMC-MOSAIC. These results demonstrate how the mixing state metrics evolve with time for several archetypal cases, each of which isolates a specific process such as coagulation, emission, or condensation. Further, we present an analysis of the mixing state evolution for a complex urban plume case, for which these processes occur simultaneously. We additionally derive theoretical properties of the mixing state index and present a family of generalized mixing state indexes that vary in the importance assigned to low-mass-fraction species.

Description: Long-term observations of positive cluster ion concentration, sources and sinks at the high altitude site of the Puy de Dôme Atmospheric Chemistry and Physics Discussions, 13, 14927-14975, 2013 Author(s): C. Rose, J. Boulon, M. Hervo, H. Holmgren, E. Asmi, M. Ramonet, P. Laj, and K. Sellegri Cluster particles (0.8–1.9 nm) are key entities involved in nucleation and new particle formation processes in the atmosphere. Cluster ions were characterized in clear sky conditions at the Puy de Dôme station (1465 m a.s.l). The studied dataset spread over five years (February 2007–February 2012), which provided a unique chance to catch seasonal variations of cluster ion properties at high altitude. Statistical values of the cluster ion concentration and diameter are reported for both positive and negative polarities. Cluster ions were found to be ubiquitous at the Puy de Dôme and displayed an annual variation with lower concentrations in spring. Positive cluster ions were less numerous than negative ones but were larger in diameters. Negative cluster ion properties seemed insensitive to the occurrence of a new particle formation (NPF) event while positive cluster ions appeared to be significantly more numerous and larger on event days. The parameters of the balance equation for the positive cluster concentration are reported, separately for the different seasons and for the NPF event days and non-event days. The steady state assumption suggests that the ionization rate is balanced with two sinks which are the ion recombination and the attachment on aerosol particles, referred as "aerosol ion sink". The aerosol ion sink was found to be higher during the warm season and dominated the loss of ions. The positive ionization rates derived from the balance equation were well correlated with the ionization rates obtained from radon measurement, and they were on average higher in summer and fall compared to winter and spring. Neither the aerosol ion sink nor the ionization rate were found to be significantly different on event days compared to non-event days, and thus they were not able to explain the different positive cluster concentrations between event and non-event days. Hence, the excess of positive small ions on event days may derive from an additional source of ions coupled with the fact that the steady state was not verified on event days.

Description: Review and uncertainty assessment of size-resolved scavenging coefficient formulations for snow scavenging of atmospheric aerosols Atmospheric Chemistry and Physics Discussions, 13, 14823-14869, 2013 Author(s): L. Zhang, X. Wang, M. D. Moran, and J. Feng Theoretical parameterizations for the size-resolved scavenging coefficient for atmospheric aerosol particles scavenged by snow (Λ snow ) need assumptions regarding (i) snow particle–aerosol particle collection efficiency E , (ii) snow particle size distribution N ( D p ), (iii) snow particle terminal velocity V D , and (iv) snow particle cross-sectional area A . Existing formulas for these parameters are reviewed in the present study and uncertainties in Λ snow caused by various combinations of these parameters are assessed. Different formulations of E can cause uncertainties in Λ snow of more than one order of magnitude for all aerosol sizes for typical snowfall intensities. E is the largest source of uncertainty among all the input parameters, similar to rain scavenging of atmospheric aerosols (Λ rain ) as was found in a previous study by Wang et al. (2010). However, other parameters can also cause significant uncertainties in Λ snow , and the uncertainties from these parameters are much larger than for Λ rain . Specifically, different N ( D p ) formulations can cause one-order-of-magnitude uncertainties in Λ snow for all aerosol sizes, as is also the case for a combination of uncertainties from both V D and A . In comparison, uncertainties in Λ rain from N ( D p ) are smaller than a factor of 5 and those from V D are smaller than a factor of 2. Λ snow estimated from one empirical formula generated from field measurements falls in the upper range of, or is slightly higher than, theoretically estimated values. The predicted aerosol concentrations obtained using different Λ snow formulas can differ by a factor of two for just a one-centimeter snowfall (liquid water equivalent of approximately 1 mm). It is likely that, for typical rain and snow event the removal of atmospheric aerosol particles by snow is more effective than removal by rain for equivalent precipitation amounts, although a firm conclusion requires much more evidence.

Description: Atmospheric waves as scaling, turbulent phenomena Atmospheric Chemistry and Physics Discussions, 13, 14797-14822, 2013 Author(s): J. Pinel and S. Lovejoy It is paradoxical that while atmospheric dynamics are highly nonlinear and turbulent that atmospheric waves are commonly modelled by linear or weakly nonlinear theories. We postulate that the laws governing atmospheric waves are on the contrary high Reynold's number ( Re ), emergent laws so that – in common with the emergent high Re turbulent laws – they are also constrained by scaling symmetries. We propose an effective turbulence – wave propagator which corresponds to a fractional and anisotropic extension of the classical wave equation propagator with dispersion relations similar to those of inertial gravity waves (and Kelvin waves) yet with an anomalous (fractional) order H wav /2. Using geostationary IR radiances, we estimate the parameters finding that H wav /2 ≈ 0.17 ± 0.04 (the classical value = 2).

Description: Optimizing Saharan dust CALIPSO retrievals Atmospheric Chemistry and Physics Discussions, 13, 14749-14795, 2013 Author(s): V. Amiridis, U. Wandinger, E. Marinou, E. Giannakaki, A. Tsekeri, S. Basart, S. Kazadzis, A. Gkikas, M. Taylor, J. Baldasano, and A. Ansmann We demonstrate improvements in CALIPSO dust extinction retrievals over North Africa and Europe when corrections are applied regarding the Saharan dust lidar ratio assumption, the separation of dust portion in detected dust mixtures, and the averaging scheme introduced in the Level 3 CALIPSO product. First, a universal, spatially constant lidar ratio of 58 sr instead of 40 sr is applied to individual Level 2 dust-related backscatter products. The resulting aerosol optical depths show an improvement compared with synchronous and co-located AERONET measurements. An absolute bias of the order of −0.03 has been found, improving on the statistically significant biases of the order of −0.10 reported in the literature for the original CALIPSO product. When compared with the MODIS co-located AOD product, the CALIPSO negative bias is even less for the lidar ratio of 58 sr. After introducing the new lidar ratio for the domain studied, we examine potential improvements to the climatological CALIPSO Level 3 extinction product: (1) by introducing a new methodology for the calculation of pure dust extinction from dust mixtures and (2) by applying an averaging scheme that includes zero extinction values for the non-dust aerosol types detected. The scheme is applied at a horizontal spatial resolution of 1° × 1° for ease of comparison with the instantaneous and co-located dust extinction profiles simulated by the BSC-DREAM8b dust model. Comparisons show that the extinction profiles retrieved with the proposed methodology reproduce the well-known model biases per sub-region examined. The very good agreement of the proposed CALIPSO extinction product with respect to AERONET, MODIS and the BSC-DREAM8b dust model, makes this dataset an ideal candidate for the provision of an accurate and robust multi-year dust climatology over North Africa and Europe.

Description: Climatology of pure Tropospheric profiles and column contents of ozone and carbon monoxide using MOZAIC in the mid-northern latitudes (24° N to 50° N) from 1994 to 2009 Atmospheric Chemistry and Physics Discussions, 13, 14695-14747, 2013 Author(s): R. M. Zbinden, V. Thouret, P. Ricaud, F. Carminati, J.-P. Cammas, and P. Nédélec The objective of this paper is to deliver the most accurate ozone (O 3 ) and carbon monoxide (CO) climatology for the pure troposphere only, i.e. exclusively from the ground to the dynamical tropopause on an individual profile basis. The results (profiles and columns) are derived solely from the M easurements of OZ one and water vapour by in-service AI rbus air C raft programme (MOZAIC) over fifteen years (1994–2009). The study, focused on the northern mid-latitudes [24° N–50° N] and [120° W–140° E], includes more than 40 000 profiles over 11 sites to give a quasi-global zonal picture. Considering all the sites, the pure tropospheric column peak-to-peak seasonal cycle ranges are 23.7–43.2 DU for O 3 and 1.7–6.9 × 10 18 mol cm −2 for CO. The maxima of the seasonal cycles are not in phase, occurring in February–April for CO and May–July for O 3 . The phase shift is related to the photochemistry and OH removal efficiencies. The purely tropospheric seasonal profiles are characterized by a typical autumn-winter/spring-summer O 3 dichotomy, (except in Los Angeles, Eastmed – a cluster of Cairo and Tel Aviv – and the regions impacted by the summer monsoon) and a summer-autumn/winter-spring CO dichotomy. We revisit the boundary-layer, mid-tropospheric (MT) and upper-tropospheric (UT) partial columns, using a new monthly-varying MT ceiling. Interestingly, the seasonal cycle maximum of the UT partial columns is shifted from summer to spring for O 3 and to very early spring for CO. Conversely, the MT maximum is shifted from spring to summer and is associated with a summer (winter) MT thickening (thinning). Lastly, the pure tropospheric seasonal cycles derived from our analysis are consistent with the cycles derived from spaceborne measurements, the correlation coefficients being r = 0.6–0.9 for O 3 , and r 〉 0.9 for CO. The cycles observed from space are nevertheless greater than MOZAIC for O 3 (by 9–18 DU) and smaller for CO (up to 1 × 10 18 mol cm −2 ). The larger winter O 3 difference between the two data sets suggests probable stratospheric contamination in satellite data due to the tropopause position. The study underlines the importance of rigorously discriminating between the stratospheric and tropospheric reservoirs and avoiding use of a monthly-averaged tropopause position without this strict discrimination, in order to assess the pure O 3 and CO tropospheric trends.

Description: A climatology of formation conditions for aerodynamic contrails Atmospheric Chemistry and Physics Discussions, 13, 14667-14693, 2013 Author(s): K. Gierens and F. Dilger Aerodynamic contrails are defined in this paper as line shaped ice clouds caused by aerodynamically triggered cooling over the wings of an aircraft in cruise which become visible immediately at the trailing edge of the wing or close to it. Effects at low altitudes like condensation to liquid droplets and their potential heterogeneous freezing are excluded from our definition. We study atmospheric conditions that allow formation of aerodynamic contrails. These conditions are stated and then applied to atmospheric data, first to a special case where an aerodynamic contrail was actually observed and then to a full year of global reanalysis data. We show where, when (seasonal variation), and how frequently (probability) aerodynamic contrails can form, and how this relates to actual patterns of air traffic. We study the formation of persistent aerodynamic contrails as well. Finally we check whether aerodynamic and exhaust contrails can coexist in the atmosphere. We show that visible aerodynamic contrails are possible only in an altitude range between roughly 540 and 250 hPa, and that the ambient temperature is the most important parameter, not the relative humidity. Finally we give an argument for our believe that currently aerodynamic contrails have a much smaller climate effect than exhaust contrails, which may however change in future with more air traffic in the tropics.

Description: Expansion of global drylands under a warming climate Atmospheric Chemistry and Physics Discussions, 13, 14637-14665, 2013 Author(s): S. Feng and Q. Fu Global drylands encompassing hyper-arid, arid, semiarid, and dry subhumid areas cover about 41% of the earth's terrestrial surface and are home to more than a third of the world's population. By analyzing observations for 1948–2008 and climate model simulations for 1948–2100, we show that global drylands have expanded in last sixty years and will continue to expand in the 21st century. By the end of this century, the world's drylands under a high greenhouse gas emission scenario are projected to be 5.8 × 10 6 km 2 (or 10%) larger than in the 1961–1990 climatology. The major expansion of arid regions will occur over southwest North America, the northern fringe of Africa, southern Africa, and Australia, while major expansions of semiarid regions will occur over the north side of the Mediterranean, southern Africa, and North and South America. The global dryland expansions will increase the population affected by water scarcity and land degradations.

Description: The role of horizontal model resolution in assessing the transport of CO in a middle latitude cyclone using WRF-Chem Atmospheric Chemistry and Physics Discussions, 13, 14871-14925, 2013 Author(s): C. A. Klich and H. E. Fuelberg We use the Weather Research and Forecasting with Chemistry (WRF-Chem) online chemical transport model to simulate a middle latitude cyclone in East Asia at three different horizontal resolutions (45, 15, and 5 km grid spacing). The cyclone contains a typical warm conveyor belt (WCB) with an embedded squall line that passes through an area having large surface concentrations (〉400 ppbv) of carbon monoxide (CO). Model output from WRF-Chem is used to compare differences between the large-scale CO vertical transport by the WCB (the 45 km simulation) with the smaller-scale transport due to its convection (the 5 km simulation). Forward trajectories are calculated from WRF-Chem output using HYSPLIT. At 45 km grid spacing, the WCB exhibits gradual ascent, lofting surface CO to 6–7 km. Upon reaching the warm front, the WCB and associated CO ascend more rapidly and later turn eastward over the Pacific Ocean. Convective transport at 5 km resolution with explicitly resolved convection occurs much more rapidly, with surface CO lofted to altitudes greater than 10 km in 1 h or less. We also compute CO vertical mass fluxes to compare differences in transport due to the different grid spacings. Upward CO flux exceeds 110 000 t h −1 in the domain with explicit convection when the squall line is at peak intensity, while fluxes from the two coarser resolutions are an order of magnitude smaller. Specific areas of interest within the 5 km domain are defined to compare the magnitude of convective transport to that within the entire 5 km region. Although convection encompasses only a small portion of the 5 km domain, it is responsible for ~40% of the upward CO transport. We also examine the vertical transport due to a short wave trough and its associated area of convection, not related to the cyclone, that lofts CO to the upper troposphere. Results indicate that fine-scale resolution with explicitly resolved convection is important when assessing the vertical transport of surface emissions in areas of deep convection.

Description: A Lagrangian view of ozone production tendency in North American outflow in summers 2009 and 2010 Atmospheric Chemistry and Physics Discussions, 13, 15141-15190, 2013 Author(s): B. Zhang, R. C. Owen, J. A. Perlinger, A. Kumar, S. Wu, M. Val Martin, L. Kramer, D. Helmig, and R. E. Honrath The Pico Mountain Observatory, located at 2225 m a.s.l. in the Azores Islands, was established in 2001 to observe long-range transport from North America to the central North Atlantic. In previous research conducted at the Observatory, ozone enhancement (〉55 ppbv) in North American outflows was observed, and efficient ozone production in these outflows was postulated. This study is focused on determining the causes for high d [O 3 ]/ d [CO] values (~1 ppbv ppbv −1 ) observed in summers of 2009 and 2010. The folded retroplume technique, developed by Owen and Honrath (2009), was applied to combine upwind FLEXPART transport pathways with GEOS-Chem chemical fields. This folded result provides a semi-Lagrangian view of polluted North American outflow in terms of physical properties and chemical processes, including production/loss rate of ozone and NO x produced by lightning and thermal decomposition of PAN. Two transport events from North America were identified for detailed analysis. High d [O 3 ]/ d [CO] was observed in both events, but due to differing transport mechanisms, ozone production tendency differed between the two. A layer of net ozone production was found at 2 km a.s.l. over the Azores in the first event plume, apparently driven by PAN decomposition during subsidence of air mass in the Azores-Bermuda High. In the second event, net ozone loss occurred during transport in the lower free troposphere, yet observed d [O 3 ]/ d [CO] was high. We estimate that in both events, CO loss through oxidation contributed significantly to d [O 3 ]/ d [CO] enhancement. Thus, CO is not appropriately used as a passive tracer of pollution in these events. In general, use of d [O 3 ]/ d [CO] as an indicator of net ozone production/loss may be invalid for any situation in which oxidants are elevated. Based on our analysis, use of d [O 3 ]/ d [CO] to diagnose ozone enhancement without verifying the assumption of negligible CO loss is not advisable.

Description: Free troposphere ozone and carbon monoxide over the North Atlantic for 2001–2011 Atmospheric Chemistry and Physics Discussions, 13, 15377-15407, 2013 Author(s): A. Kumar, S. Wu, M. F. Weise, R. Honrath, R. C. Owen, D. Helmig, L. Kramer, M. Val Martin, and Q. Li In-situ measurements of carbon monoxide (CO) and ozone (O 3 ) at the Pico Mountain Observatory (PMO) located in the Azores, Portugal are analyzed together with results from atmospheric chemical transport modeling (GEOS-Chem) and satellite remote sensing (AIRS for CO and TES for O 3 ) to examine the evolution of free-troposphere CO and O 3 over the North Atlantic for 2001–2011. GEOS-Chem captured the seasonal cycles for CO and O 3 well but significantly underestimated the mixing ratios of CO, particularly in spring. Statistically significant (using a significance level of 0.05) decreasing trends were found for both CO and O 3 based on harmonic regression analysis of the measurement data. The best estimates of the trend for CO and O 3 measurements are −0.31 ± 0.30 (2-σ) ppbv yr −1 and −0.21 ± 0.11 (2-σ) ppbv yr −1 , respectively. Similar decreasing trends for both species were obtained with GEOS-Chem simulation results. The major factor contributing to the reported decrease in CO and O 3 mixing ratios at PMO over the past decade is the decline in anthropogenic CO and O 3 -precursor emissions in regions such as North America and Europe. The increase in Asian emissions does not seem to outweigh the impact of these declines resulting in overall decreasing trends for both CO and O 3 . For O 3 , however, increase in atmospheric water vapor content associated with climate change also appears to be a contributing factor causing enhanced destruction of the O 3 during transport from source regions. These hypotheses are supported by results from the GEOS-Chem tagged CO and tagged O 3 simulations.

Description: Mesoscale modeling of smoke transport over the Southeast Asian Maritime Continent: coupling of smoke direct radiative feedbacks below and above the low-level clouds Atmospheric Chemistry and Physics Discussions, 13, 15443-15492, 2013 Author(s): C. Ge, J. Wang, and J. S. Reid The online-coupled Weather Research and Forecasting model with Chemistry (WRF-Chem) is used to simulate the direct and semi-direct radiative impacts of smoke particles over the Southeast Asian Marine Continents (MC, 10° S–10° N, 90° E–150° E) during October 2006 when a significant El Nino event caused the highest biomass burning activity since 1997. With the use of OC (Organic Carbon)/BC (Black Carbon) ratio of 10 in the smoke emission inventory, the baseline simulation shows that the low-level clouds amplifying effect on smoke absorption led to a warming effect at the top-of-atmosphere (TOA) with a domain/monthly average forcing value of ~20 W m −2 over the islands of Borneo and Sumatra. The smoke-induced monthly average daytime heating (0.3 K) that is largely confined above the low-level clouds results in the local convergence over the smoke source region. This heating-induced convergence coupled with daytime planetary boundary layer turbulent mixing, transports more smoke particles above the planetary boundary layer height (PBLH), hence rendering a positive feedback. This positive feedback contrasts with the decrease of cloud fraction resulted from the combined effects of smoke heating within the cloud layer and the more stability in the boundary layer; the latter can be considered as a negative feedback in which decrease of cloud fraction weakens the heating by smoke particles above the clouds. During nighttime, the elevated smoke layer (above clouds in daytime) is decoupled from boundary layer, and the reduction of PBLH due to the residual surface cooling from the daytime lead to the accumulation of smoke particles near the surface. Because of smoke radiative extinction, on monthly basis, the amount of the solar input at the surface is reduced as large as 60 W m −2 , which lead to the decrease of sensible heat, latent heat, 2 m air temperature, and PBLH by a maximum of 20 W m −2 , 20 W m −2 , 1 K, 120 m, respectively. The decrease of boundary layer mixing and the generation of convergence above the PBL also results in a reduction of precipitable water 1–2 km above the PBLH and more precipitable water near the surface and in upper part of the middle troposphere with changes around 0.1 mm. Overall, there is less of a change of column water vapor over the land, and an increase of water vapor amount over the Karimata Strait. The cloud changes over continents are mostly occurred over the islands of Sumatra and Borneo during the daytime, where the low-level cloud fraction decreases more than 10%. However, the change of local wind (include sea breeze) induced by the smoke radiative feedback leads to more convergence over Karimata Strait and south coastal area of Kalimantan during both daytime and night time; consequently, cloud fraction is increased there up to 20%. The sensitivities with different OC/BC ratio show the importance of the smoke single scattering albedo for the smoke semi-direct effects. A case study on 31 October 2006 further demonstrated a much larger (more than twice of the monthly average) feedback induced by smoke aerosols. The decreased sea breeze during big events can lead to prominent increase (40%) of low-level cloud over coastal water. Lastly, the direct and semi-direct radiative impact of smoke particles over the Southeast Asian Marine Continents is summarized as a conceptual model.

Description: Global carbon monoxide products from combined AIRS, TES and MLS measurements on A-train satellites Atmospheric Chemistry and Physics Discussions, 13, 15409-15441, 2013 Author(s): J. X. Warner, R. Yang, Z. Wei, F. Carminati, A. Tangborn, Z. Sun, W. Lahoz, J.-L. Attié, L. El Amraoui, and B. Duncan This study tests a novel methodology to add value to satellite datasets. This methodology, data fusion, is similar to data assimilation, except that the background model-based field is replaced by a satellite dataset, in this case AIRS (Atmospheric Infrared Sounder) carbon monoxide (CO) measurements. The observational information comes from CO measurements with lower spatial coverage than AIRS, namely, from TES (Tropospheric Emission Spectrometer) and MLS (Microwave Limb Sounder). We show that combining these datasets with data fusion uses the higher spectral resolution of TES to extend AIRS CO observational sensitivity to the lower troposphere, a region especially important for air quality studies. We also show that combined CO measurements from AIRS and MLS provide enhanced information in the UTLS (upper troposphere/lower stratosphere) region compared to each product individually. The combined AIRS/TES and AIRS/MLS CO products are validated against DACOM (differential absorption mid-IR diode laser spectrometer) in situ CO measurements from the INTEX-B (Intercontinental Chemical Transport Experiment: MILAGRO and Pacific phases) field campaign and in situ data from HIPPO (HIAPER Pole-to-Pole Observations) flights. The data fusion results show improved sensitivities in the lower and upper troposphere (20–30% and above 20%, respectively) as compared with AIRS-only retrievals, and improved coverage compared with TES and MLS CO data.

Description: An inverse modeling method to assess the source term of the Fukushima nuclear power plant accident using gamma dose rate observations Atmospheric Chemistry and Physics Discussions, 13, 15567-15614, 2013 Author(s): O. Saunier, A. Mathieu, D. Didier, M. Tombette, D. Quélo, V. Winiarek, and M. Bocquet The Chernobyl nuclear accident and more recently the Fukushima accident highlighted that the largest source of error on consequences assessment is the source term including the time evolution of the release rate and its distribution between radioisotopes. Inverse modeling methods, which combine environmental measurements and atmospheric dispersion models, have proven efficient in assessing source term due to an accidental situation (Gudiksen, 1989; Krysta and Bocquet, 2007; Stohl et al., 2012a; Winiarek et al., 2012). Most existing approaches are designed to use air sampling measurements (Winiarek et al., 2012) and some of them also use deposition measurements (Stohl et al., 2012a; Winiarek et al., 2013) but none of them uses dose rate measurements. However, it is the most widespread measurement system, and in the event of a nuclear accident, these data constitute the main source of measurements of the plume and radioactive fallout during releases. This paper proposes a method to use dose rate measurements as part of an inverse modeling approach to assess source terms. The method is proven efficient and reliable when applied to the accident at the Fukushima Daiichi nuclear power plant (FD-NPP). The emissions for the eight main isotopes 133 Xe, 134 Cs, 136 Cs, 137 Cs, 137m Ba, 131 I, 132 I and 132 Te have been assessed. Accordingly, 103 PBq of 131 I, 35.5 PBq of 132 I, 15.5 PBq of 137 Cs and 12 100 PBq of noble gases were released. The events at FD-NPP (such as venting, explosions, etc.) known to have caused atmospheric releases are well identified in the retrieved source term. The estimated source term is validated by comparing simulations of atmospheric dispersion and deposition with environmental observations. The result is that the model-measurement agreement for all of the monitoring locations is correct for 80% of simulated dose rates that are within a factor of 2 of the observed values. Changes in dose rates over time have been overall properly reconstructed, especially in the most contaminated areas to the northwest and south of the FD-NPP. A comparison with observed atmospheric activity concentration and surface deposition shows that the emissions of caesiums and 131 I are realistic but that 132 I and 132 Te are probably underestimated and noble gases are likely overestimated. Finally, an important outcome of this study is that the method proved to be perfectly suited to emergency management and could contribute to improve emergency response in the event of a nuclear accident.

Description: Forcing of stratospheric chemistry and dynamics during the Dalton Minimum Atmospheric Chemistry and Physics Discussions, 13, 15061-15104, 2013 Author(s): J. G. Anet, S. Muthers, E. Rozanov, C. C. Raible, T. Peter, A. Stenke, A. I. Shapiro, J. Beer, F. Steinhilber, S. Brönnimann, F. Arfeuille, Y. Brugnara, and W. Schmutz The response of atmospheric chemistry and climate to volcanic eruptions and a decrease in solar activity during the Dalton Minimum is investigated with the fully coupled atmosphere-ocean-chemistry general circulation model SOCOL-MPIOM covering the time period 1780 to 1840 AD. We carried out several sensitivity ensemble experiments to separate the effects of (i) reduced solar ultra-violet (UV) irradiance, (ii) reduced solar visible and near infrared irradiance, (iii) enhanced galactic cosmic ray intensity as well as less intensive solar energetic proton events and auroral electron precipitation, and (iv) volcanic aerosols. The introduced changes of UV irradiance and volcanic aerosols significantly influence stratospheric climate in the early 19th century, whereas changes in the visible part of the spectrum and energetic particles have smaller effects. A reduction of UV irradiance by 15% causes global ozone decrease below the stratopause reaching 8% in the midlatitudes at 5 hPa and a significant stratospheric cooling of up to 2 °C in the midstratosphere and to 6 °C in the lower mesosphere. Changes in energetic particle precipitation lead only to minor changes in the yearly averaged temperature fields in the stratosphere. Volcanic aerosols heat the tropical lower stratosphere allowing more water vapor to enter the tropical stratosphere, which, via HO x reactions, decreases upper stratospheric and mesospheric ozone by roughly 4%. Conversely, heterogeneous chemistry on aerosols reduces stratospheric NO x leading to a 12% ozone increase in the tropics, whereas a decrease in ozone of up to 5% is found over Antarctica in boreal winter. The linear superposition of the different contributions is not equivalent to the response obtained in a simulation when all forcing factors are applied during the DM – this effect is especially well visible for NO x /NO y . Thus, this study highlights the non-linear behavior of the coupled chemistry-climate system. Finally, we conclude that especially UV and volcanic eruptions dominate the changes in the ozone, temperature and dynamics while the NO x field is dominated by the EPP. Visible radiation changes have only very minor effects on both stratospheric dynamics and chemistry.

Description: Estimation of aerosol water and chemical composition from AERONET at Cabauw, the Netherlands Atmospheric Chemistry and Physics Discussions, 13, 15191-15232, 2013 Author(s): A. J. van Beelen, G. J. H. Roelofs, O. P. Hasekamp, J. S. Henzing, and T. Röckmann Remote sensing of aerosols provides important information on the atmospheric aerosol abundance. However, due to the hygroscopic nature of aerosol particles observed aerosol optical properties are influenced by atmospheric humidity, and the measurements do not unambiguously characterize the aerosol dry mass and composition which complicates the comparison with aerosol models. In this study we derive aerosol water and chemical composition by a modeling approach that combines individual measurements of remotely sensed aerosol properties (e.g. optical thickness, single scattering albedo, refractive index and size distribution) from an AERONET (Aerosol Robotic Network) sun-photometer with radiosonde measurements of relative humidity. The model simulates water uptake by aerosols based on the chemical composition and size distribution. A minimization method is used to calculate aerosol composition and concentration, which are then compared to in situ measurements from the Intensive Measurement Campaign At the Cabauw Tower (IMPACT, May 2008, the Netherlands). Computed concentrations show reasonable agreement with surface observations and follow the day-to-day variability in observations. Total dry mass (33 ± 12 μg m −3 ) and black carbon concentrations (0.7 ± 0.3 μg m −3 ) are generally accurately computed. The uncertainty in the AERONET (real) refractive index (0.025–0.05) introduces larger uncertainty in the modeled aerosol composition (e.g. sulfates, ammonium nitrate or organic matter) and leads to an uncertainty of 0.1–0.25 in aerosol water volume fraction. Water volume fraction is highly variable depending on composition, up to 〉0.5 at 70–80% and

Description: Evaluation of various methods to measure particulate bound mercury and associated artifacts Atmospheric Chemistry and Physics Discussions, 13, 8585-8614, 2013 Author(s): S. Wang, T. M. Holsen, J. Huang, and Y.-J. Han This study was performed to determine how sampling artifacts associated with various sampling methods including open faced filter (OFF) pack, micro orifice uniform deposit impactor (MOUDI), and Tekran speciation system (TekSpec) impact particulate bound mercury (PBM) measurements. PBM measured by the MOUDI for 48 h was statistically lower than that measured with the TekSpec every 2 h, indicating that negative artifacts were significant for long sampling durations. Negative artifacts were also identified in lab experiments as the Hg 0 and HgCl 2 concentrations associated with particulate matter on the filter significantly decreased when the filter was exposed to zero air. Positive artifacts were also investigated. The OFF sampling for 48 h, which is likely to be associated with both positive and negative artifacts, measured a significantly lower PBM concentration than the TekSpec while the OFF and MOUDI (48 h sampling – minimal positive artifacts) showed similar results, suggesting that positive artifacts were minor under the rural condition encountered (low atmospheric gaseous oxidized mercury and typical oxidants concentrations). The Hg speciation associated with particles varied with atmospheric temperature, with the contribution of less volatile species including HgO and HgS increasing and more volatile Hg 0 and HgCl 2 decreasing as atmospheric temperature increased. There was significant correlation for PBM larger than 2.5 μm between TekSpec frit and MOUDI in this study, indicating that TekSpec frit is a good alternative sampler for measuring the concentration of coarse PBM.

Description: Aerosol and precipitation chemistry in the southwestern United States: spatiotemporal trends and interrelationships Atmospheric Chemistry and Physics Discussions, 13, 8615-8662, 2013 Author(s): A. Sorooshian, T. Shingler, A. Harpold, C. W. Feagles, T. Meixner, and P. D. Brooks This study characterizes the spatial and temporal patterns of aerosol and precipitation composition at six sites across the United States Southwest between 1995 and 2010. Precipitation accumulation occurs mostly during the wintertime (December–February) and during the monsoon season (July–September). Rain and snow pH levels are usually between 5–6, with crustal-derived species playing a major role in acid neutralization. These species (Ca 2+ , Mg 2+ , K + ,Na + ) exhibit their highest concentrations between March and June in both PM 2.5 and precipitation due mostly to dust. Crustal-derived species concentrations in precipitation exhibit positive relationships with SO 4 2− , NO 3 − , and Cl − , suggesting that acidic gases likely react with and partition to either crustal particles or hydrometeors enriched with crustal constituents. Concentrations of particulate SO 4 2− show a statistically significant correlation with rain SO 4 2− unlike snow SO 4 2− , which may be related to some combination of the vertical distribution of SO 4 2− (and precursors) and the varying degree to which SO 4 2− -enriched particles act as cloud condensation nuclei versus ice nuclei in the region. The coarse : fine aerosol mass ratio was correlated with crustal species concentrations in snow unlike rain, suggestive of a preferential role of coarse particles (mainly dust) as ice nuclei in the region. Precipitation NO 3 − : SO 4 2− ratios exhibit the following features with potential explanations discussed: (i) they are higher in precipitation as compared to PM 2.5 ; (ii) they exhibit the opposite annual cycle compared to particulate NO 3 − : SO 4 2− ratios; and (iii) they are higher in snow relative to rain during the wintertime. Long-term trend analysis for the monsoon season shows that the NO 3 − : SO 4 2− ratio in rain decreased at the majority of sites due mostly to air pollution regulations of SO 4 2− precursors.

Description: Effects of relative humidity on aerosol light scattering: results from different European sites Atmospheric Chemistry and Physics Discussions, 13, 8939-8984, 2013 Author(s): P. Zieger, R. Fierz-Schmidhauser, E. Weingartner, and U. Baltensperger The effect of aerosol water uptake on the aerosol particle light scattering coefficient (σ sp ) is described in this study by comparing measurements from five European sites: the Jungfraujoch, located in the Swiss Alps at 3580 m a.s.l., Ny-Ålesund, located on Spitsbergen in the Arctic, Mace Head, a coastal site in Ireland, Cabauw, a rural site in the Netherlands and Melpitz, a regional background site in Eastern Germany. These sites were selected according to the aerosol type usually encountered at that location. The scattering enhancement factor f (RH,λ) is the key parameter to describe the effect of water uptake on the particle light scattering. It is defined as the σ sp (RH) at a certain relative humidity (RH) and wavelength λ divided by its dry value. f (RH) largely varied at the five sites starting from very low values of f (RH = 85%,λ = 550 nm) around 1.28 for mineral dust to 3.41 for Arctic aerosol. Hysteresis behavior was observed at all sites except at the Jungfraujoch due to the absence of sea salt. Closure studies and Mie simulations showed that both size and chemical composition determine the magnitude of f (RH). Both parameters are also needed to successfully predict f (RH). Finally, the measurement results were compared to the widely used aerosol model OPAC (Hess et al., 1998). Significant discrepancies were seen especially at intermediate RH ranges, which were mainly attributed to inappropriate implemented hygroscopic growth within OPAC. Replacement of the hygroscopic growth with recent literature values showed a clear improvement of the OPAC model.

Description: Spectro-microscopic measurements of carbonaceous aerosol aging in Central California Atmospheric Chemistry and Physics Discussions, 13, 9179-9216, 2013 Author(s): R. C. Moffet, T. C. Rödel, S. T. Kelly, X. Y. Yu, G. T. Carroll, J. Fast, R. A. Zaveri, A. Laskin, and M. K. Gilles Carbonaceous aerosols are responsible for large uncertainties in climate models, degraded visibility, and adverse health effects. The Carbonaceous Aerosols and Radiative Effects Study (CARES) was designed to study carbonaceous aerosols in the natural environment of Central Valley, California, and learn more about their atmospheric formation and aging. This paper presents results from spectro-microscopic measurements of carbonaceous particles collected during CARES at the time of pollution accumulation event (27–29 June 2010), when in situ measurements indicated an increase in the organic carbon content of aerosols as the Sacramento urban plume aged. Computer controlled scanning electron microscopy coupled with an energy dispersive X-ray detector (CCSEM/EDX) and scanning transmission X-ray microscopy coupled with near edge X-ray absorption spectroscopy (STXM/NEXAFS) were used to probe the chemical composition and morphology of individual particles. It was found that the mass of organic carbon on individual particles increased through condensation of secondary organic aerosol. STXM/NEXAFS indicated that the number fraction of homogenous organic particles lacking inorganic inclusions (greater than ~50 nm diameter) increased with plume age as did the organic mass per particle. Comparison of the CARES spectro-microscopic data set with a similar dataset obtained in Mexico City during the MILAGRO campaign showed that individual particles in Mexico City contained twice as much carbon as those sampled during CARES. The number fraction of soot particles at the Mexico City urban site (30%) was larger than at the CARES urban site (10%) and the most aged samples from CARES contained less carbon-carbon double bonds. Differences between carbonaceous particles in Mexico City and California result from different sources, photochemical conditions, gas phase reactants, and secondary organic aerosol precursors. The detailed results provided by these spectro-microscopic measurements will allow for a comprehensive evaluation of aerosol process models used in climate research.

Description: Laboratory studies of immersion and deposition mode ice nucleation of ozone aged mineral dust particles Atmospheric Chemistry and Physics Discussions, 13, 8701-8767, 2013 Author(s): Z. A. Kanji, A. Welti, C. Chou, O. Stetzer, and U. Lohmann Ice nucleation in the atmosphere is central to the understanding the microphysical properties of mixed-phase and cirrus clouds. Ambient conditions such as temperature ( T ) and relative humidity (RH), as well as aerosol properties such as chemical composition and mixing state play an important role in predicting ice formation in the troposphere. Previous field studies have reported the absence of sulphate and organic compounds on mineral dust ice crystal residuals sampled at mountain top stations or aircraft based measurements despite the long range transport mineral dust is subjected to. We present laboratory studies of ice nucleation for immersion and deposition mode on ozone aged mineral dust particles for 233 〈 T 〈 263 K that will represent ageing but not internal mixing with in(organic) compounds. Heterogeneous ice nucleation of untreated kaolinite (Ka) and Arizona Test Dust (ATD) particles is compared to corresponding aged particles that are subjected to ozone exposures of 0.4–4.3 ppmv in a stainless steel aerosol tank. The portable ice nucleation counter (PINC) and immersion chamber combined with the Zurich ice nucleation chamber (IMCA – ZINC) are used to conduct deposition and immersion mode measurements respectively. Ice active fractions as well as ice active surface site densities ( n s ) are reported and observed to increase as a function of temperature. We present first results that demonstrate enhancement of the ice nucleation ability of aged mineral dust particles in both the deposition and immersion mode due to ageing. Additionally, these are also the first results to show a suppression of heterogeneous ice nucleation without the condensation of a coating of (in)organic material. In immersion mode, low exposure Ka particles showed enhanced ice activity requiring a median freezing temperature of 1.5 K warmer than that of untreated Ka whereas high exposure ATD particles showed suppressed ice nucleation requiring a median freezing temperature of 3 K colder than that of untreated ATD. In deposition mode, low exposure Ka had ice active fractions of an order of magnitude higher than untreated Ka, where as high exposure ATD had ice active fractions up to a factor of 4 lower than untreated ATD. Based on our results, we present parameterizations in terms of n s ( T ) that can represent ice nucleation of atmospherically aged and non-aged particles for both immersion and deposition mode. We find excellent agreement (to within less than a factor of 2) with field measurements when parameterizations derived from our results are used to predict ice nuclei concentrations in the troposphere.

Description: Retrieval of the Eyjafjallajökull volcanic aerosol optical and microphysical properties from POLDER/PARASOL measurements Atmospheric Chemistry and Physics Discussions, 13, 8663-8699, 2013 Total and polarized radiances provided by the Polarization and Directionality of Earth Reflectances (POLDER) satellite sensor are used to retrieve the microphysical and optical properties of the volcanic plume observed during the Eyjafjallajökull volcano eruption in 2010, over cloud-free and cloudy ocean scenes. We selected two plume conditions, fresh aerosols near the sources (three cases) and a downwind volcanic plume observed over the North Sea 30 h after its injection into the atmosphere (aged aerosols). In the near-source conditions, the aerosol properties depend on the distance to the plume. Within the plume, aerosols are mainly non-spherical and in the coarse mode with an effective radius equal to 1.50 (± 0.15) μm and an Ångström Exponent (AE) close to 0.0. Far from the plume, in addition to the coarse mode, there are smaller particles retrieved in the accumulation mode suggesting a mixture of sulfate aerosols and volcanic dust, resulting in an AE around 0.8. The properties of the aerosols also depend on whether the plume is fresh or aged. For the downwind (aged) plume, if non-spherical coarse particles as well as some fine mode particles are still retrieved, the AE is smaller, around ~ 0.4. In addition, the real refractive index (RR) values are larger for the downwind plume (1.42 〈 RR 〈 1.58) than for the near-source plume (1.38 〈 RR 〈 1.48). The mean Single Scattering Albedo (SSA) retrieved at 0.865 μm was estimated at 0.97 over some parts of the downwind and near-source plumes; despite the low accuracy of our retrievals, the derived SSA values suggest that the ash particles are rather absorbing. To consider the particle shape, a combination of spheroid models is used. Although the employed model enabled accurate modeling of the POLDER signal in case of non-spherical ash, our approach failed to model the signal over the optically thickest parts of the near-source plume. The most probable reason for this is speculated to be the presence of ice crystals within the plume. For the Aerosol Above Clouds (AAC) scenes, polarized measurements allowed the retrieval of the Optical Thickness (OT) and the AE of optically thin volcanic ash. We found that all the cloud parameters retrieved by passive sensors were biased due to the presence of the elevated volcanic plumes. Finally, thermal infrared measurements were used to identify the type of multi-layer scene (i.e. cirrus clouds or volcanic dust above liquid clouds) and the retrieval method also provided the OT of thin cirrus layers above the clouds near Iceland.

Description: Present and future nitrogen deposition to national parks in the United States: critical load exceedances Atmospheric Chemistry and Physics Discussions, 13, 9151-9178, 2013 Author(s): R. A. Ellis, D. J. Jacob, M. Payer, L. Zhang, C. D. Holmes, B. A. Schichtel, T. Blett, E. Porter, L. H. Pardo, and J. A. Lynch National parks in the United States are protected areas wherein the natural habitat is to be conserved for future generations. Deposition of anthropogenic nitrogen (N) transported from areas of human activity (fuel combustion, agriculture) may affect these natural habitats if it exceeds an ecosystem-dependent critical load (CL). We quantify and interpret the deposition to Class I US national parks for present-day and future (2050) conditions using the GEOS-Chem global chemical transport model with 1/2° × 2/3° horizontal resolution over North America. We estimate CL values in the range 2.5–5 kg N ha −1 yr −1 for the different parks with the goal of protecting the most sensitive ecosystem receptors. For present-day conditions, we find 24 out of 45 parks to be in CL exceedance and 14 more to be marginally so. Many of these are in remote areas of the West. Most (40–85%) of the deposition originates from NO x emissions (fuel combustion). We then project future changes in N deposition using the Representative Concentration Pathway (RCP) emission scenarios for 2050. These feature 52–73% declines in US NO x emissions relative to present but 19–50% increases in US ammonia (NH 3 ) emissions. Nitrogen deposition at US national parks then becomes dominated by domestic NH 3 emissions. While deposition decreases in the East relative to present, there is little progress in the West and increases in some regions. We find that 17–25 US national parks will have CL exceedances in 2050 based on the RCP scenarios. Even in total absence of anthropogenic NO x emissions, 14–18 parks would still have a CL exceedance. Returning all parks to N deposition below CL by 2050 will require at least a 55% decrease in anthropogenic NH 3 emissions relative to RCP-projected 2050 levels.

Description: Redox activity of naphthalene secondary organic aerosol Atmospheric Chemistry and Physics Discussions, 13, 9107-9149, 2013 Author(s): R. D. McWhinney, S. Zhou, and J. P. D. Abbatt Chamber secondary organic aerosol (SOA) from low-NO x photooxidation of naphthalene by hydroxyl radical was examined with respect to its redox cycling behaviour using the dithiothreitol (DTT) assay. Naphthalene SOA was highly redox active, consuming DTT at an average rate of 118 ± 14 pmol per minute per μg of SOA material. Measured particle-phase masses of the major previously identified redox active products, 1,2- and 1,4-naphthoquinone, accounted for only 21 ± 3% of the observed redox cycling activity. The redox-active 5-hydroxy-1,4-naphthoquinone was identified as a new minor product of naphthalene oxidation, and including this species in redox activity predictions increased the predicted DTT reactivity to 30 ± 5% of observations. Similar attempts to predict redox behaviour of oxidised two-stroke engine exhaust particles by measuring 1,2-naphthoquinone, 1,4-naphthoquinone and 9,10-phenanthrenequinone predicted DTT decay rates only 4.9 ± 2.5% of those observed. Together, these results suggest that there are substantial unidentified redox-active SOA constituents beyond the small quinones that may be important toxic components of these particles. A gas-to-SOA particle partitioning coefficient was calculated to be (7.0 ± 2.5) × 10 −4 m 3 μg −1 for 1,4-naphthoquinone at 25 °C. This value suggests that under typical warm conditions, 1,4-naphthoquinone is unlikely to contribute strongly to redox behaviour of ambient particles, although further work is needed to determine the potential impact under conditions such as low temperatures where partitioning to the particle is more favourable. As well, higher order oxidation products that likely account for a substantial fraction of the redox cycling capability of the naphthalene SOA are likely to partition much more strongly to the particle phase.

Description: Factors that influence surface PM 2.5 values inferred from satellite observations: perspective gained for the Baltimore-Washington Area during DISCOVER-AQ Atmospheric Chemistry and Physics Discussions, 13, 23421-23459, 2013 Author(s): S. Crumeyrolle, G. Chen, L. Ziemba, A. Beyersdorf, L. Thornhill, E. Winstead, R. Moore, M. A. Shook, and B. Anderson During the NASA DISCOVER-AQ campaign over the Washington D.C., - Baltimore, MD, metropolitan region in July 2011, the NASA P-3B aircraft performed extensive profiling of aerosol optical, chemical, and microphysical properties. These in-situ profiles were coincident with ground based remote sensing (AERONET) and in-situ (PM 2.5 ) measurements. Here, we use this data set to study the correlation between the PM 2.5 observations at the surface and the column integrated measurements. Aerosol optical depth (AOD) calculated with the extinction (532 nm) measured during the in-situ profiles was found to be strongly correlated with the volume of aerosols present in the boundary layer (BL). Despite the strong correlation, some variability remains, and we find that the presence of aerosol layers above the BL (in the buffer layer – BuL) introduces a significant uncertainties in PM 2.5 estimates based on column-integrated measurements. This motivates the use of active remote sensing techniques to dramatically improve air quality retrievals. Since more than a quarter of the AOD values observed during DISCOVER-AQ are dominated by aerosol water uptake, the f (RH) amb (obtained from two nephelometers at different relative humidities – RHs) is used to study the impact of the aerosol hygroscopicity. The results indicate that PM 2.5 can be predicted within a factor of 1.6 even when the vertical variability of the f (RH) amb is assumed to be negligible.

Description: On clocks and clouds Atmospheric Chemistry and Physics Discussions, 13, 23461-23490, 2013 Author(s): M. K. Witte, P. Y. Chuang, and G. Feingold Cumulus clouds exhibit a life cycle that consists of: (a) the growth phase (increasing size, most notably in the vertical direction); (b) the mature phase (growth ceases; any precipitation that develops is strongest during this period); and (c) the dissipation phase (cloud dissipates because of precipitation and/or entrainment; no more dynamical support). Although radar can track clouds over time and give some sense of the age of a cloud, most aircraft in situ measurements lack temporal context. We use large eddy simulations of trade wind cumulus cloud fields from cases during the Barbados Oceanographic and Meteorological Experiment (BOMEX) and Rain In Cumulus over the Ocean (RICO) campaigns to demonstrate a potential cumulus cloud "clock". We find that the volume-averaged total water mixing ratio r t is a useful cloud clock for the 12 clouds studied. A cloud's initial r t is set by the subcloud mixed-layer mean r t and decreases monotonically from the initial value due primarily to entrainment. The clock is insensitive to aerosol loading, environmental sounding and extrinsic cloud properties such as lifetime and volume. In some cases (more commonly for larger clouds), multiple pulses of buoyancy occur, which complicate the cumulus clock by replenishing r t . The clock is most effectively used to classify clouds by life phase.

Description: Assessment of the effect of air pollution controls on trends in shortwave radiation over the United States from 1995 through 2010 from multiple observation networks Atmospheric Chemistry and Physics Discussions, 13, 23719-23755, 2013 Author(s): C.-M. Gan, J. Pleim, R. Mathur, C. Hogrefe, C. N. Long, J. Xing, S. Roselle, and C. Wei Long term datasets of all-sky and clear-sky downwelling shortwave (SW) radiation, cloud cover fraction and aerosol optical depth (AOD) are analyzed together with surface concentration from several networks (e.g. SURFRAD, CASTNET, IMPROVE and ARM) in the United States (US). Seven states with varying climatology are selected to better understand the effects of aerosols and clouds on SW radiation. This analysis aims to assess the effects of reductions in anthropogenic aerosol burden resulting from substantial reductions in emissions of sulfur dioxide (SO 2 ) and nitrogen oxides (NO x ) over the past 16 yr across the US on trends in SW radiation. The SO 2 and NO x emission data show decreasing trends from 1995 to 2010 which indirectly validates the effects of the Clean Air Act (CAA) in the US. Meanwhile, the total column AOD and surface total PM 2.5 observations also show decreasing trends in the eastern US but slightly increasing trends in the western US. Moreover, measured surface concentrations of several other pollutants (i.e. SO 2 , SO 4 and NO x ) have the same behavior as the AOD and total PM 2.5 . First, all-sky downwelling SW radiation is assessed together with the cloud cover. Results of this analysis show strong increasing trends in all-sky downwelling SW radiation with decreasing trends in cloud cover. However, since observations of both all-sky direct and diffuse SW radiation are increasing, there may be other factors contributing to the radiation trends in addition to the decreasing trends in overall cloud cover. To investigate the role of direct radiative effects of aerosols, clear-sky downwelling radiation is analyzed so that cloud effects are eliminated. However, similar increasing trends in clear-sky direct and diffuse SW radiation are observed. While significantly decreasing trends in AOD and surface concentration along with increasing SW radiation (both all-sky and clear-sky) in the eastern US during 1995–2010 imply the occurrence of direct aerosol mediated "brightening", the increasing trends of both all-sky and clear sky diffuse SW radiation contradicts this conclusion since diffuse radiation would be expected to decrease as aerosols direct effects decrease. After investigating several confounding factors, the increasing trend in diffuse SW may be due to more high-level cirrus from increasing air traffic over the US. In contrast to the eastern US, radiation observations in the western US do not show any indication of "brightening" which is consistent with the observations (e.g. AOD, PM 2.5 and surface concentration) that show the aerosol loading increasing slightly. This outcome is not unexpected because the CAA controls were mainly aimed at reducing air pollutants emission in the eastern US and air pollutant level in the western US are much lower.

Description: Modeled global effects of airborne desert dust on air quality and premature mortality Atmospheric Chemistry and Physics Discussions, 13, 24023-24050, 2013 Author(s): D. Giannadaki, A. Pozzer, and J. Lelieveld Fine particulate matter is one of the most important factors contributing to air pollution. Epidemiological studies have related increased levels of atmospheric particulate matter to premature human mortality caused by cardiopulmonary disease and lung cancer. However, a limited number of investigations have focused on the contribution of airborne desert dust particles. Here we assess the effects of dust particles with an aerodynamic diameter smaller than 2.5 μm (DU 2.5 ) on human mortality for the year 2005. We used the EMAC atmospheric chemistry general circulation model at high resolution to simulate global atmospheric dust concentrations. We applied a health impact function to estimate premature mortality for the global population of 30 yr and older, using parameters from epidemiological studies. We estimate a global cardiopulmonary mortality of about 402 thousand and about 10 thousand by lung cancer in 2005. The associated years of life lost are about 3.47 million and 96 thousand per year due to cardiopulmonary disease and lung cancer, respectively. We estimate the global fraction of the cardiopulmonary and lung cancer deaths caused by atmospheric desert dust to be about 1.7%, though in the 20 countries most affected by dust this is much higher, about 15–50%. These countries are primarily found in the so-called "dust belt" from North Africa across the Middle East and South Asia to East Asia.

Description: Aerosols optical and physical characteristics and direct radiative forcing during a "Shamal" dust storm, a case study Atmospheric Chemistry and Physics Discussions, 13, 23895-23941, 2013 Author(s): T. M. Saeed, H. Al-Dashti, and C. Spyrou Dust aerosols are analyzed for their optical and physical properties during an episode of dust storm that hit Kuwait on 26 March 2003 when "Iraqi Freedom" military operation was in full swing. The intensity of the dust storm was such that it left a thick suspension of dust throughout the following day, 27 March, resulting in a considerable cooling effect at the surface on both days. Ground-based measurements of aerosol optical thickness reached 3.617 and 4.17 on 26–27 March respectively while Ångstrom coefficient, α 870/440 , dropped to −0.0234 and −0.0318. Particulate matter concentration of diameter 10 μm or less, PM 10 , peaked at 4800 μg m −3 during dust storm hours of 26 March. Moderate resolution imaging spectrometer (MODIS) retrieved optical and physical characteristics that exhibited extreme values as well. The synoptic of the dust storm is presented and source regions are identified using total ozone mapping spectrometer (TOMS) aerosol index retrieved images. The vertical profile of the dust layer was simulated using SKIRON atmospheric model. Instantaneous net direct radiative forcing is calculated at top of atmosphere (TOA) and surface level. The thick dust layer of 26 March resulted in cooling the TOA by −60 Wm −2 and surface level by −175 Wm −2 for a surface albedo of 0.35. Slightly higher values were obtained for 27 March due to the increase in aerosol optical thickness. The large reduction in the radiative flux at the surface level had caused a drop in surface temperature by approximately 6 °C below its average value. Radiative heating/cooling rates in the shortwave and longwave bands were also examined. Shortwave heating rate reached a maximum value of 2 °K day −1 between 3 and 5 km, dropped to 1.5 °K day −1 at 6 km and diminished at 8 km. Longwave radiation initially heated the lower atmosphere by a maximum value of 0.2 °K day −1 at surface level, declined sharply at increasing altitude and diminished at 4 km. Above 4 km longwave radiation started to cool the atmosphere slightly reaching a maximum rate of −0.1 °K day −1 at 6 km.

Description: A scale and aerosol aware stochastic convective parameterization for weather and air quality modeling Atmospheric Chemistry and Physics Discussions, 13, 23845-23893, 2013 Author(s): G. A. Grell and S. R. Freitas A convective parameterization is described and evaluated that may be used in high resolution non-hydrostatic mesoscale models as well as in modeling systems with unstructured varying grid resolutions and for convection aware simulations. This scheme is based on a stochastic approach originally implemented by Grell and Devenyi (2002). Two approaches are tested on resolutions ranging from 20 to 5 km. One approach is based on spreading subsidence to neighboring grid points, the other one on a recently introduced method by Arakawa et al. (2011). Results from model intercomparisons, as well as verification with observations indicate that both the spreading of the subsidence and Arakawa's approach work well for the highest resolution runs. Because of its simplicity and its capability for an automatic smooth transition as the resolution is increased, Arakawa's approach may be preferred. Additionally, interactions with aerosols have been implemented through a CCN dependent autoconversion of cloud water to rain as well as an aerosol dependent evaporation of cloud drops. Initial tests with this newly implemented aerosol approach show plausible results with a decrease in predicted precipitation in some areas, caused by the changed autoconversion mechanism. This change also causes a significant increase of cloud water and ice detrainment near the cloud tops. Some areas also experience an increase of precipitation, most likely caused by strengthened downdrafts.

Description: Climatology of new particle formation events in the subtropical North Atlantic free troposphere at Izaña GAW observatory Atmospheric Chemistry and Physics Discussions, 13, 24127-24169, 2013 Author(s): M. I. García, S. Rodríguez, Y. González, and R. D. García A climatology of new particle formation (NPF) events in the subtropical North Atlantic free troposphere is presented. A four year data set (June 2008–June 2012), which includes number size distributions (10–600 nm), reactive gases (SO 2 , NO x , and O 3 ), several components of solar radiation and meteorological parameters, measured at Izaña Global Atmospheric Watch observatory (2400 m above sea level; Tenerife, Canary Islands) was analysed. On average, NPF occurred during 30% of the days,the mean values of the formation and growth rates during the study period were 0.49 cm −3 s −1 and 0.42 nm h −1 , correspondingly. There is a clearly marked NPF season (May to August), when these events account for 50 to 60% of the days/month. Monthly mean values of the formation and growth rates exhibit higher values during this season (0.50–0.95 cm −3 s −1 and 0.48–0.58 nm h −1 , respectively) than during other periods. The two steps (formation and growth) of the NPF process mostly occur under the prevailing northern winds typical of this region. Sulphur dioxide and UV radiation show higher levels during NPF events than in other type of episodes. The presence of Saharan dust in the free troposphere is associated with a decrease in the formation rates of new particles. In the analysis of the year-to-year variability, mean sulphur dioxide concentration (within the range 60–300 ppt) was the parameter that exhibited the highest correlation with the frequency of NPF episodes. The availability of this trace gas (i.e. their oxidation products) seems also to have a influence on the duration of the events, number of formed nucleation particles, formation rates and growth rates. We identified a set of NPF events in which two nucleation modes (that may evolve at different rates) occur simultaneously and for which further investigations are necessary.

Description: Numerical analysis of the chemical kinetic mechanisms of ozone depletion and halogen release in the polar troposphere Atmospheric Chemistry and Physics Discussions, 13, 24171-24222, 2013 Author(s): L. Cao, H. Sihler, U. Platt, and E. Gutheil In recent years, the role of halogen species (e.g. Br, Cl) in the troposphere of polar regions is investigated after the discovery of their importance for boundary layer ozone destruction in the polar spring. Halogen species take part in an auto-catalytic chemical cycle including key self reactions. In this study, several chemical reaction schemes are investigated, and the importance of specific reactions and their rate constants is identified by a sensitivity analysis. A category of heterogeneous reactions related to HOBr activate halogen ions from sea salt aerosols, fresh sea ice or snow pack, driving the "bromine explosion". In the Arctic, a small amount of NO x may exist, which comes from nitrate contained in the snow, and this NO x may have a strong impact on ozone depletion. The heterogeneous reaction rates are parameterized by considering the aerodynamic resistance, a reactive surface ratio, β, i.e. ratio of reactive surface area to total ground surface area, and the boundary layer height, L mix . It is found that for β = 1, the ozone depletion process starts after five days and lasts for 40 h for L mix = 200 m. Ozone depletion duration becomes independent of the height of the boundary layer for about β≥20, and it approaches a value of two days for β=100. The role of nitrogen and chlorine containing species on the ozone depletion rate is studied. The calculation of the time integrated bromine and chlorine atom concentrations suggests a value in the order of 10 3 for the [Br] / [Cl] ratio, which reveals that atomic chlorine radicals have minor direct influence on the ozone depletion. The NO x concentrations are influenced by different chemical cycles over different time periods. During ozone depletion, the reaction cycle involving the BrONO 2 hydrolysis is dominant. A critical value of 0.002 of the uptake coefficient of the BrONO 2 hydrolysis reaction at the aerosol and saline surfaces is identified, beyond which the existence of NO x species accelerate the ozone depletion event – for lower values, deceleration occurs.

Description: Modeling the influence of precursor volatility and molecular structure on secondary organic aerosol formation using evaporated fuel experiments Atmospheric Chemistry and Physics Discussions, 13, 24405-24434, 2013 Author(s): S. H. Jathar, N. M. Donahue, P. J. Adams, and A. L. Robinson We use SOA production data from an ensemble of evaporated fuels to test various SOA formation models. Except for gasoline, traditional SOA models focusing exclusively on volatile species in the fuels under-predict the observed SOA formation. These models can be improved dramatically by accounting for lower volatility species, but at the cost of a large set of free parameters. In contrast, a SOA model based only on the volatility of the precursor, starting with the volatility distribution of the evaporated fuels and optimized for the volatility reduction of first-generation products, reasonably reproduces the observed SOA formation with relatively few free parameters. The exceptions are exotic fuels such as Fischer-Tropsch fuels that expose the central assumption of the volatility based model that most emissions consist of complex mixtures displaying reasonably average behavior. However, for the vast majority of fuels, the volatility based model performs well.

Description: Secondary organic aerosol production from diesel vehicle exhaust: impact of aftertreatment, fuel chemistry and driving cycle Atmospheric Chemistry and Physics Discussions, 13, 24223-24262, 2013 Author(s): T. D. Gordon, A. A. Presto, N. T. Nguyen, W. H. Robertson, K. Na, K. N. Sahay, M. Zhang, C. Maddox, P. Rieger, S. Chattopadhyay, H. Maldonado, M. M. Maricq, and A. L. Robinson Environmental chamber ("smog chamber") experiments were conducted to investigate secondary organic aerosol (SOA) production from dilute emissions from two medium-duty diesel vehicles (MDDVs) and three heavy-duty diesel vehicles (HDDVs) under urban-like conditions. Some of the vehicles were equipped with emission control aftertreatment devices including diesel particulate filters (DPF), selective catalytic reduction (SCR) and diesel oxidation catalysts (DOC). Experiments were also performed with different fuels (100% biodiesel and low-, medium- or high-aromatic ultralow sulfur diesel) and driving cycles (Unified Cycle, Urban Dynamometer Driving Schedule, and creep+idle). During normal operation, vehicles with a catalyzed DPF emitted very little primary particulate matter (PM). Furthermore, photo-oxidation of dilute emissions from these vehicles produced essentially no SOA (below detection limit). However, significant primary PM emissions and SOA production were measured during active DPF regeneration experiments. Nevertheless, under reasonable assumptions about DPF regeneration frequency, the contribution of regeneration emissions to the total vehicle emissions is negligible, reducing PM trapping efficiency by less than 2%. Therefore, catalyzed DPFs appear to be very effective in reducing both primary and secondary fine particulate matter from diesel vehicles. For both MDDVs and HDDVs without aftertreatment substantial SOA formed in the smog chamber – with the emissions from some vehicles generating twice as much SOA as primary organic aerosol after three hours of oxidation at typical urban VOC : NO x ratios (3:1). Comprehensive organic gas speciation was performed on these emissions, but less than half of the measured SOA could be explained by traditional (speciated) SOA precursors. The remainder presumably originates from the large fraction (~30%) of the non-methane organic gas emissions that could not be speciated using traditional one-dimensional gas-chromatography. The unspeciated organics – likely comprising less volatile species, such as intermediate volatility organic compounds – appear to be important SOA precursors; we estimate that the effective SOA yield (defined as the ratio of SOA mass to reacted precursor mass) was 9 ± 6% if both speciated SOA precursors and unspeciated organics are included in the analysis. SOA production from creep+idle operation was 3–4 times larger than SOA production from the same vehicle operated over the Urban Dynamometer Driving Schedule (UDDS). Fuel properties had little or no effect on primary PM emissions or SOA formation.

Description: Primary to secondary organic aerosol: evolution of organic emissions from mobile combustion sources Atmospheric Chemistry and Physics Discussions, 13, 24263-24300, 2013 Author(s): A. A. Presto, T. D. Gordon, and A. L. Robinson A series of smog chamber experiments were conducted to investigate the transformation of primary organic aerosol (POA) and formation of secondary organic aerosol (SOA) during the photo-oxidation of dilute gasoline and diesel motor vehicle exhaust. In half of the experiments POA was present in the chamber at the onset of photo-oxidation. In these experiments positive matrix factorization (PMF) was used to determine separate POA and SOA factors from aerosol mass spectrometer data. A two-factor solution, with one POA factor and one SOA factor, was sufficient to describe the organic aerosol in all but one experiment. In the other half of the experiments, POA was not present at the onset of photo-oxidation; these experiments are considered "pure SOA" experiments. The POA mass spectrum was similar to the mass spectra of the hydrocarbon-like organic aerosol factor determined from ambient datasets with one exception, a diesel vehicle equipped with a diesel oxidation catalyst. The SOA in all experiments had a constant composition over the course of photo-oxidation, and did not appear to age with continued oxidation. The SOA mass spectra for the various gasoline and diesel vehicles were similar to each other, but markedly different than ambient oxidized organic aerosol factors. Van Krevelen analysis of the POA and SOA factors for gasoline and diesel experiments reveal slopes of −0.68 and −0.43, respectively. This suggests that the oxidation chemistry in these experiments is a combination of carboxylic acid and alcohol/peroxide formation, consistent with ambient oxidation chemistry. These experiments also provide insight to the mixing behavior of the POA and SOA. Analysis of the time series of the POA factor concentration and a basis-set model both indicate that for all but one of the vehicles tested here, the POA and SOA seem to mix and form a single organic aerosol phase.

Description: CARIBIC DOAS observations of nitrous acid and formaldehyde in a large convective cloud Atmospheric Chemistry and Physics Discussions, 13, 24343-24403, 2013 Author(s): K.-P. Heue, H. Riede, D. Walter, C. A. M. Brenninkmeijer, T. Wagner, U. Frieß, U. Platt, A. Zahn, G. Stratmann, and H. Ziereis The CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) flying laboratory measures once per month the chemical composition at cruise altitude (10...12 km) during 4 consecutive Lufthansa flights. Here we present a case study of enhanced nitrogen oxides (NO x ), nitrous acid (HONO), and formaldehyde (HCHO) in a thunderstorm cloud over the Caribbean islands of Guadeloupe in August 2011. Nitrous acid is an important reservoir gas for OH radicals, and only few observations of HONO at cruise altitude exist. CARIBIC is designed as a long period atmospheric observation system, the actual system has been flying almost monthly since 8 yr now. During this period only very few similar events (one since 2008) were observed. Due to multiple scattering the light path inside clouds is enhanced, thereby lowering the detection limit of the DOAS instrument. Under background conditions the detection limits are 46 ppt for HONO, 387 ppt for \chem{HCHO}, and 100 ppt for NO 2 and are roughly three times lower inside the cloud. Based on radiative transfer simulations we estimate the path length to 90{\ldots}100 km and the cloud top height to ≈15 km. The inferred mixing ratios of HONO, HCHO and NO 2 are 37 ppt, 400 ppt and 170 ppt, respectively. Bromine monoxide (BrO) remained below the detection limit of 1 ppt. Because the uplifted air masses originated from the remote marine boundary layer and lightning was observed in the area by the World Wide Lightning Location Network several hours prior to the measurement, the NO (≈1.5 ppb) enhancement was in all likelihood caused by lightning. The main source for the observed HCHO is probably updraught from the boundary layer, because the chemical formation of formaldehyde due to methane oxidation is too weak. Besides HCHO also CH 3 OOH and isoprene are considered as precursors. The chemical box model CAABA is used to estimate the \chem{NO} and HCHO source strengths, which are necessary to explain our measurements. For NO a source strength of 8.25 × 10 9 molec cm −2 s −1 km −1 is found, which corresponds to the lightning activity as observed by the World Wide Lightning Location network and a lightning emission of 4.2 × 10 25 NO molec/flash. The HCHO updraught is of the order of 121 × 10 9 molec cm −2 s −1 km −1 . Also isoprene and CH 3 OOH as possible HCHO sources were studied and similar source strengths were found.

Description: Oligomer formation within secondary organic aerosol: equilibrium and dynamic considerations Atmospheric Chemistry and Physics Discussions, 13, 24605-24634, 2013 Author(s): E. R. Trump and N. M. Donahue We present a model based on the volatility basis set to consider the potential influence of oligomer content on volatility-driven SOA yields. The implications for aerosol evaporation studies, including dilution, chamber thermo-equilibration, and thermodenuder studies are also considered. A simplified description of oligomer formation reproduces essentially all of the broad classes of equilibrium and dynamical observations related to SOA formation and evaporation: significant oligomer content may be consistent with mass yields that increase with organic aerosol mass concentration; reversible oligomerization can explain the hysteresis between the rate of SOA formation and its evaporation rate upon dilution; and the model is consistent with both chamber thermo-equilibration studies and thermodenuder studies of SOA evaporation.

Description: Factors controlling variability in the oxidative capacity of the troposphere since the Last Glacial Maximum Atmospheric Chemistry and Physics Discussions, 13, 24517-24603, 2013 Author(s): L. T. Murray, L. J. Mickley, J. O. Kaplan, E. D. Sofen, M. Pfeiffer, and B. Alexander The oxidative capacity of past atmospheres is highly uncertain. We present here a new climate-biosphere-chemistry modeling framework to determine oxidant levels in the present and past troposphere. We use the GEOS-Chem chemical transport model driven by meteorological fields from the NASA Goddard Institute of Space Studies (GISS) ModelE, with land cover and fire emissions from dynamic global vegetation models. We present time-slice simulations for the present day, late preindustrial (AD 1770), and the Last Glacial Maximum (LGM; 19–23 ka), and we test the sensitivity of model results to uncertainty in lightning and fire emissions. We find that most preindustrial and paleo climate simulations yield reduced oxidant levels relative to the present day. Contrary to prior studies, tropospheric mean OH in our ensemble shows little change at the LGM relative to the preindustrial (0.5 ± 12%), despite large reductions in methane concentrations. We find a simple linear relationship between tropospheric mean ozone photolysis rates, water vapor, and total emissions of NO x and reactive carbon that explains 72% of the variability in global mean OH in 11 different simulations across the last glacial-interglacial time interval and the Industrial Era. Key parameters controlling the tropospheric oxidative capacity over glacial-interglacial periods include overhead stratospheric ozone, tropospheric water vapor, and lightning NO x emissions. Variability in global mean OH since the LGM is insensitive to fire emissions. Our simulations are broadly consistent with ice-core records of Δ 17 O in sulfate and nitrate at the LGM, and CO, HCHO, and H 2 O 2 in the preindustrial. Our results imply that the glacial-interglacial changes in atmospheric methane observed in ice cores are predominantly driven by changes in its sources as opposed to its sink with OH.

Description: Characterizing the impact of urban emissions on regional aerosol particles; airborne measurements during the MEGAPOLI experiment Atmospheric Chemistry and Physics Discussions, 13, 24885-24924, 2013 Author(s): E. J. Freney, K. Sellegri, F. Canonaco, A. Colomb, A. Borbon, V. Michoud, J.-F. Doussin, S. Crumeyrolle, N. Amarouch, J.-M. Pichon, A. S. H. Prévôt, M. Beekmann, and A. Schwarzenböeck The MEGAPOLI experiment took place in July 2009. The aim of this campaign was to study the aging and reactions of aerosol and gas-phase emissions in the city of Paris. Three ground-based measurement sites and several mobile platforms including instrument equipped vehicles and the ATR-42 aircraft were involved. We present here the variations in particle- and gas-phase species over the city of Paris using a combination of high-time resolution measurements aboard the ATR-42 aircraft. Particle chemical composition was measured using a compact time-of-flight aerosol mass spectrometer (C-ToF-AMS) giving detailed information of the non-refractory submicron aerosol species. The mass concentration of BC, measured by a particle absorption soot photometer (PSAP), was used as a marker to identify the urban pollution plume boundaries. Aerosol mass concentrations and composition were affected by air-mass history, with air masses that spent longest time over land having highest fractions of organic aerosol and higher total mass concentrations. The Paris plume is mainly composed of organic aerosol (OA), black carbon and nitrate aerosol, as well as high concentrations of anthropogenic gas-phase species such as toluene, benzene, and NO x . Using BC and CO as tracers for air-mass dilution, we observe the ratio of ΔOA / ΔBC and ΔOA / ΔCO increase with increasing photochemical age (−log(NO x / NO y ). Plotting the equivalent ratios for the Positive Matrix Factorization (PMF) resolved species (LV-OOA, SV-OOA, and HOA) illustrate that the increase in OA is a result of secondary organic aerosol (SOA). Within Paris the changes in the ΔOA / ΔCO are similar to those observed during other studies in Mexico city, Mexico and in New England, USA. Using the measured VOCs species together with recent organic aerosol formation yields we predicted ~ 50% of the measured organics. These airborne measurements during the MEGAPOLI experiment show that urban emissions contribute to the formation of OA, and have an impact on aerosol composition on a regional scale. They provide a quantitative measure of this impact in terms of urban plume composition and evolution relative to background aerosol composition.

Description: Role of ozone in SOA formation from alkane photooxidation Atmospheric Chemistry and Physics Discussions, 13, 24713-24754, 2013 Author(s): X. Zhang, R. H. Schwantes, M. M. Coggon, C. L. Loza, K. A. Schilling, R. C. Flagan, and J. H. Seinfeld Long-chain alkanes, which can be categorized as intermediate volatile organic compounds (IVOCs), are an important source of secondary organic aerosol (SOA). Mechanisms for the gas-phase OH-initiated oxidation of long-chain alkanes have been well documented; particle-phase chemistry, however, has received less attention. The δ-hydroxycarbonyl, which is generated from the isomerization of alkoxy radicals, can undergo heterogeneous cyclization to form substituted dihydrofuran. Due to the presence of C=C bonds, the substituted dihydrofuran is predicted to be highly reactive with OH, and even more so with O 3 and NO 3 , thus opening a reaction pathway that is not usually accessible to alkanes. This work focuses on the role of substituted dihydrofuran formation and its subsequent reaction with OH, and more importantly ozone, in SOA formation from the photooxidation of long-chain alkanes. Experiments were carried out in the Caltech Environmental Chamber using dodecane as a representative alkane to investigate the difference in aerosol composition generated from "OH-oxidation dominating" vs. "ozonolysis dominating" environments. A detailed mechanism incorporating the specific gas-phase photochemistry, together with the heterogeneous formation of substituted dihydrofuran and its subsequent gas-phase OH/O 3 oxidation, is presented to evaluate the importance of this reaction channel in the dodecane SOA formation. We conclude that: (1) the formation of δ-hydroxycarbonyl and its subsequent heterogeneous conversion to substituted dihydrofuran is significant in the presence of NO x ; (2) the ozonolysis of substituted dihydrofuran dominates over the OH-initiated oxidation under conditions prevalent in urban and rural air; and (3) a spectrum of highly-oxygenated products with carboxylic acid, ester, and ether functional groups are produced from the substituted dihydrofuran chemistry, thereby affecting the average oxidation state of the SOA.

Description: A joint data assimilation system (Tan-Tracker) to simultaneously estimate surface CO 2 fluxes and 3-D atmospheric CO 2 concentrations from observations Atmospheric Chemistry and Physics Discussions, 13, 24755-24784, 2013 Author(s): X. Tian, Z. Xie, Y. Liu, Z. Cai, Y. Fu, H. Zhang, and L. Feng To quantitatively estimate CO 2 surface fluxes (CFs) from atmospheric observations, a joint data assimilation system ("Tan-Tracker") is developed by incorporating a joint data assimilation framework into the GEOS-Chem atmospheric transport model. In Tan-Tracker, we choose an identity operator as the CF dynamical model to describe the CFs' evolution, which constitutes an augmented dynamical model together with the GEOS-Chem atmospheric transport model. In this case, the large-scale vector made up of CFs and CO 2 concentrations is taken as the prognostic variable for the augmented dynamical model. And thus both CO 2 concentrations and CFs are jointly assimilated by using the atmospheric observations (e.g., the in-situ observations or satellite measurements). In contrast, in the traditional joint data assimilation frameworks, CFs are usually treated as the model parameters and form a state-parameter augmented vector jointly with CO 2 concentrations. The absence of a CF dynamical model will certainly result in a large waste of observed information since any useful information for CFs' improvement achieved by the current data assimilation procedure could not be used in the next assimilation cycle. Observing system simulation experiments (OSSEs) are carefully designed to evaluate the Tan-Tracker system in comparison to its simplified version (referred to as TT-S) with only CFs taken as the prognostic variables. It is found that our Tan-Tracker system is capable of outperforming TT-S with higher assimilation precision for both CO 2 concentrations and CO 2 fluxes, mainly due to the simultaneous assimilation of CO 2 concentrations and CFs in our Tan-Tracker data assimilation system.

Description: AERONET-based microphysical and optical properties of smoke-dominated aerosol near source regions and transported over oceans, and implications for satellite retrievals of aerosol optical depth Atmospheric Chemistry and Physics Discussions, 13, 25013-25065, 2013 Author(s): A. M. Sayer, N. C. Hsu, T. F. Eck, A. Smirnov, and B. N. Holben Smoke aerosols from biomass burning are an important component of the global aerosol cycle. Analysis of Aerosol Robotic Network (AERONET) retrievals of size distribution and refractive index reveals variety between biomass burning aerosols in different global source regions, in terms of aerosol particle size and single scatter albedo (SSA). Case studies of smoke transported to coastal/island AERONET sites also mostly lie within the range of variability at near-source sites. Two broad ''families'' of aerosol properties are found, corresponding to sites dominated by boreal forest burning (larger, broader fine mode, with midvisible SSA ∼0.95), and those influenced by grass, shrub, or crop burning with additional forest contributions (smaller, narrower particles with SSA ∼0.88–0.9 in the midvisible). The strongest absorption is seen in southern African savannah at Mongu (Zambia), with average SSA ∼0.85 in the midvisible. These can serve as candidate sets of aerosol microphysical/optical properties for use in satellite aerosol optical depth (AOD) retrieval algorithms. The models presently adopted by these algorithms over ocean are often insufficiently absorbing to represent these biomass burning aerosols. A corollary of this is an underestimate of AOD in smoke outflow regions, which has important consequences for applications of these satellite datasets.

Description: Henry's law constants of diacids and hydroxypolyacids: recommended values Atmospheric Chemistry and Physics Discussions, 13, 25125-25156, 2013 Author(s): S. Compernolle and J.-F. Müller In spite of the importance of diacids and functionalised diacids for organic aerosol formation through aqueous-phase processes in droplets and aerosol water, there seems to be no reliable set of experimental values for their Henry's law constants (HLC). We show that their estimation through the use of infinite dilution activity coefficients is also prone to error. Here we present HLC values for diacids and hydroxy polyacids determined from solubilities, water activities and vapour pressures of solids or solutions, by employing thermodynamic relationships. The vapour pressures are found to be the largest source of error, but the analysis of the obtained HLC points to inconsistencies among specific vapour pressure data sets. Although there is considerable uncertainty, the HLC of diacids appear to be higher than estimated by the often cited review work of Saxena and Hildemann (1996).

Description: High concentrations of biological aerosol particles and ice nuclei during and after rain Atmospheric Chemistry and Physics Discussions, 13, 1767-1793, 2013 Author(s): J. A. Huffman, C. Pöhlker, A. J. Prenni, P. J. DeMott, R. H. Mason, N. H. Robinson, J. Fröhlich-Nowoisky, Y. Tobo, V. R. Després, E. Garcia, D. J. Gochis, E. Harris, I. Müller-Germann, C. Ruzene, B. Schmer, B. Sinha, D. A. Day, M. O. Andreae, J. L. Jimenez, M. Gallagher, S. M. Kreidenweis, A. K. Bertram, and U. Pöschl Bioaerosols are relevant for public health and may play an important role in the climate system, but their atmospheric abundance, properties and sources are not well understood. Here we show that the concentration of airborne biological particles in a forest ecosystem increases dramatically during rain and that bioparticles are closely correlated with atmospheric ice nuclei (IN). The greatest increase of bioparticles and IN occurred in the size range of 2–6 μm, which is characteristic for bacterial aggregates and fungal spores. By DNA analysis we found high diversities of airborne bacteria and fungi, including human and plant pathogens (mildew, smut and rust fungi, molds, Enterobacteraceae, Pseudomonadaceae ). In addition to known bacterial and fungal IN ( Pseudomonas sp., Fusarium sporotrichioides ), we discovered two species of IN-active fungi that were not previously known as biological ice nucleators ( Isaria farinosa and Acremonium implicatum ). Our findings suggest that atmospheric bioaerosols, IN and rainfall are more tightly coupled than previously assumed.

Description: Modelling the effects of gravity waves on stratocumulus clouds observed during VOCALS-UK Atmospheric Chemistry and Physics Discussions, 13, 1717-1765, 2013 Author(s): P. J. Connolly, G. Vaughan, P. Cook, G. Allen, H. Coe, T. W. Choularton, C. Dearden, and A. Hill During the VOCALS campaign spaceborne satellite observations showed that travelling gravity wave packets, generated by geostrophic adjustment, resulted in perturbations to marine boundary layer clouds over the south east Pacific ocean. Often, these perturbations were reversible in that passage of the wave resulted in the clouds becoming brighter (in the wave crest) then darker (in the wave trough) and subsequently recovering their properties after the passage of the wave. However, occasionally the wave packets triggered irreversible changes to the clouds, which transformed from closed mesoscale cellular convection to open form. In this paper we use large eddy simulation (LES) to examine the physical mechanisms that cause this transition. Specifically, we examine whether the clearing of the cloud is due to: (i) the wave causing additional cloud-top entrainment of warm, dry air or (ii) whether the additional condensation of liquid water onto the existing drops and the subsequent formation of drizzle are the important mechanisms. We find that although the wave does cause additional drizzle formation, this is not the reason for the persistent clearing of the cloud; rather it is the additional entrainment of warm, dry air into the cloud, although this only has a significant effect when the cloud is starting to de-couple from the boundary layer. The result in this case is a change from a stratocumulus to a more cumulus-like regime. For the simulations presented here, cloud condensation nuclei scavenging did not play an important role in the clearing of the cloud. The results have implications for understanding transitions between the different cellular regimes in marine boundary layer clouds.

Description: Observations of peroxyacetyl nitrate (PAN) in the upper troposphere by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) Atmospheric Chemistry and Physics Discussions, 13, 1575-1607, 2013 Author(s): K. A. Tereszchuk, D. P. Moore, J. J. Harrison, C. D. Boone, M. Park, J. J. Remedios, W. J. Randel, and P. F. Bernath Peroxyacetyl nitrate (CH 3 CO·O 2 NO 2 , abbreviated as PAN) is a trace molecular species present in the troposphere and lower stratosphere due primarily to pollution from fuel combustion and the pyrogenic outflows from biomass burning. In the lower troposphere, PAN has a relatively short life-time and is principally destroyed within a few hours through thermolysis, but it can act as a reservoir and carrier of NO x in the colder temperatures of the upper troposphere where UV photolysis becomes the dominant loss mechanism. Pyroconvective updrafts from large biomass burning events can inject PAN into the upper troposphere and lower stratosphere (UTLS), providing a means for the long-range transport of NO x . Given the extended lifetimes at these higher altitudes, PAN is readily detectable via satellite remote sensing. A new PAN data product is now available for the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) Version 3.0 data set. We report measurements of PAN in Boreal biomass burning plumes recorded during the Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites (BORTAS) campaign. The retrieval method employed and errors analysis are described in full detail. The retrieved volume mixing ratio (VMR) profiles are compared to coincident measurements made by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument on the European Space Agency (ESA) ENVIronmental SATellite (ENVISAT). Three ACE-FTS occultations containing measurements of Boreal biomass burning outflows, recorded during BORTAS, were identified as having coincident measurements with MIPAS. In each case, the MIPAS measurements demonstrated good agreement with the ACE-FTS VMR profiles for PAN. The ACE-FTS PAN data set is used to obtain zonal mean distributions of seasonal averages from ~5 to 20 km. A strong seasonality is clearly observed for PAN concentrations in the global UTLS. Since the principal source of PAN in the UTLS is due to lofted biomass burning emissions from the pyroconvective updrafts created by large fires, the observed seasonality in enhanced PAN coincides with fire activity in different geographical regions throughout the year. This work is part of an in-depth investigation that is being conducted in an effort to study the aging and chemical evolution of biomass burning emissions in the UTLS by remote, space-borne measurements made by ACE-FTS to further our understanding of the impact of pyrogenic emissions on atmospheric chemistry. Included in this study will be the addition of new, pyrogenic, volatile organic hydrocarbons (VOCs) and oxygenated volatile organic compounds (OVOCs) to expand upon the already extensive suite of molecules retrieved by ACE-FTS to aid in elucidating biomass burning plume chemistry in the free troposphere.

Description: Modeling microphysical effects of entrainment in clouds observed during EUCAARI-IMPACT field campaign Atmospheric Chemistry and Physics Discussions, 13, 1489-1526, 2013 Author(s): D. Jarecka, H. Pawlowska, W. W. Grabowski, and A. A. Wyszogrodzki This paper discusses aircraft observations and large-eddy simulation (LES) of the 15 May 2008, North Sea boundary-layer clouds from the EUCAARI-IMPACT field campaign. These clouds were advected from the north-east by the prevailing lower-tropspheric winds, and featured stratocumulus-over-cumulus cloud formations. Almost-solid stratocumulus deck in the upper part of the relatively deep weakly decoupled marine boundary layer overlaid a field of small cumuli with a cloud fraction of ~10%. The two cloud formations featured distinct microphysical characteristics that were in general agreement with numerous past observations of strongly-diluted shallow cumuli on the one hand and solid marine boundary-layer stratocumulus on the other. Macrophysical and microphysical cloud properties were reproduced well by the double-moment warm-rain microphysics large-eddy simulation. A novel feature of the model is its capability to locally predict homogeneity of the subgrid-scale mixing between the cloud and its cloud-free environment. In the double-moment warm-rain microphysics scheme, the homogeneity is controlled by a single parameter α, that ranges from 0 to 1 and limiting values representing the homogeneous and the extremely inhomogeneous mixing scenarios, respectively. Parameter α depends on the characteristic time scales of the droplet evaporation and of the turbulent homogenization. In the model, these scales are derived locally based on the subgrid-scale turbulent kinetic energy, spatial scale of cloudy filaments, the mean cloud droplet radius, and the humidity of the cloud-free air entrained into the cloud. Simulated mixing is on average quite inhomogeneous, with the mean parameter α around 0.7 across the entire depth of the cloud field, but with local variations across almost the entire range, especially near the base and the top of the cloud field.

Description: Constraints on emissions of carbon monoxide, methane, and a suite of hydrocarbons in the Colorado Front Range using observations of 14 CO 2 Atmospheric Chemistry and Physics Discussions, 13, 1609-1672, 2013 Author(s): B. W. LaFranchi, G. Pétron, J. B. Miller, S. J. Lehman, A. E. Andrews, E. J. Dlugokencky, B. R. Miller, S. A. Montzka, B. Hall, W. Neff, C. Sweeney, J. C. Turnbull, D. E. Wolfe, P. P. Tans, K. R. Gurney, and T. P. Guilderson Atmospheric radiocarbon ( 14 CO) represents an important observational constraint on emissions of fossil-fuel derived carbon into the atmosphere due to the absence of 14 CO in fossil fuel reservoirs. The high sensitivity and precision that accelerator mass spectrometry (AMS) affords in atmospheric 14 CO analysis has greatly increased the potential for using such measurements to evaluate bottom-up emissions inventories of fossil fuel CO 2 (CO 2 ff), as well as those for other co-emitted species. Here we use observations of 14 CO 2 and a series of hydrocarbons and combustion tracers from discrete air samples collected between June 2009 and September 2010 at the National Oceanic and Atmospheric Administration Boulder Atmospheric Observatory (BAO; Lat: 40.050° N, Lon: 105.004° W) to derive emission ratios of each species to CO 2 ff. From these emission ratios, we estimate emissions of these species by using the Vulcan CO 2 ff high resolution data product as a reference. The species considered in this analysis are carbon monoxide (CO), methane (CH 4 ), acetylene (C 2 H 2 ), benzene (C 6 H 6 ), and C 3 –C 5 alkanes. Comparisons of top-down emissions estimates are made to existing inventories of these species for Denver and adjacent counties, as well as to previous efforts to estimate emissions from atmospheric observations over the same area. We find that CO is overestimated in the 2008 National Emissions Inventory (NEI, 2008) by a factor of ~2. A close evaluation of the inventory suggests that the ratio of CO emitted per unit fuel burned from on-road gasoline vehicles is likely over-estimated by a factor of 2.5. The results also suggest that while the oil and gas sector is the largest contributor to the CH 4 signal in air arriving from the north and east, it is very likely that other sources, including agricultural sources, contribute to this signal and must be accounted for when attributing these signals to oil and gas industry activity from a top-down perspective. Our results are consistent with ~60% of the total CH 4 emissions from regions to the north and east of the BAO tower stemming from the oil and gas industry, equating to ~70 Gg yr −1 or ~1.7% of total natural gas production in the region.

Description: Large-eddy simulation of organized precipitating trade wind cumulus clouds Atmospheric Chemistry and Physics Discussions, 13, 1855-1889, 2013 Author(s): A. Seifert and T. Heus Trade wind cumulus clouds often organize in along-wind cloud streets and across-wind mesoscale arcs. We present a benchmark large-eddy simulation which resolves the individual clouds as well as the mesoscale organization on scales of O (10 km). Different methods to quantify organization of cloud fields are applied and discussed. Using perturbed physics large-eddy simulations experiments the processes leading to the formation of cloud clusters and the mesoscale arcs are revealed. We find that both cold pools as well as the sub-cloud layer moisture field are crucial to understand the organization of precipitating shallow convection. Further sensitivity studies show that microphysical assumptions can have a pronounced impact on the onset of cloud organization.

Description: Retrieving volcanic, desert dust, and sea-salt particle properties from two/three-component particle mixtures after long-range transport using UV-VIS polarization Lidar and T-matrix Atmospheric Chemistry and Physics Discussions, 13, 1891-1947, 2013 Author(s): G. David, B. Thomas, T. Nousiainen, A. Miffre, and P. Rairoux During transport by advection, atmospheric nonspherical particles, such as volcanic, desert dust or sea-salt particles experience several chemical and physical processes, leading to a complex vertical atmospheric layering at remote sites where intrusion episodes occur. In this contribution, a new methodology is proposed to analyze this complex vertical layering in the case of a two/three-component particle external mixtures after long-range transport. This methodology relies on a precise analysis of the spectral and polarization properties of the light backscattered by atmospheric particles. It is based on combining a sensitive and accurate UV-VIS polarization Lidar experiment with accurate T-matrix numerical simulations and air mass back-trajectories. The Lyon UV-VIS polarization Lidar is used to efficiently partition the particle mixture into its nonspherical components, while the T-matrix algorithm is used for computing backscattering and depolarization properties specific to nonspherical volcanic, desert dust and sea-salt particles, the latter being described in the cubic shape approximation. It is shown that, after long-range transport, the particle mixtures' depolarization ratio δ p differs from the nonspherical particles' depolarization ratio δ ns due to the presence of spherical particles in the mixture. Hence, after identifying a tracer for nonspherical particles, particle backscattering coefficients specific to each nonspherical component can be retrieved in a two component external mixture. For three-component mixtures, the spectral properties of light must in addition be addressed by using a dual-wavelength polarization Lidar. Hence, for the first time, in a three-component external mixture, the nonsphericity of each particle is taken into account in a so-called 2β + 2δ formalism. Applications of this new methodology are then demonstrated in two case studies carried out in Lyon, France, related to the mixing of Eyjafjallajökull volcanic ash with sulphate particles (case of a two-component mixture) and to the mixing of dust with sea-salt and water-soluble particles (case of a three-component mixture). This new methodology, which is able to provide separate vertical profiles of mixed atmospheric dust, sea-salt and water-soluble particles, may be useful for accurate radiative forcing assessments.

Description: Ozone photochemistry in boreal biomass burning plumes Atmospheric Chemistry and Physics Discussions, 13, 1795-1853, 2013 Author(s): M. Parrington, P. I. Palmer, A. C. Lewis, J. D. Lee, A. R. Rickard, P. Di Carlo, J. W. Taylor, J. R. Hopkins, S. Punjabi, D. E. Oram, G. Forster, E. Aruffo, S. J. Moller, S. J.-B. Bauguitte, J. D. Allan, H. Coe, and R. J. Leigh We present an analysis of ozone photochemistry observed by aircraft measurements of boreal biomass burning plumes over Eastern Canada in the summer of 2011. Measurements of ozone and a number of key chemical species associated with ozone photochemistry, including non-methane hydrocarbons (NMHCs), nitrogen oxides (NO x ) and total nitrogen containing species (NO y ), were made from the UK FAAM BAe-146 research aircraft as part of the quantifying the impact of BOReal forest fires on tropospheric oxidants over the Atlantic using Aircraft and Satellites (BORTAS) experiment between 12 July and 3 August 2011. We found that ozone mixing ratios measured in biomass burning plumes were indistinguishable from non-plume measurements, but evaluating them in relationship to measurements of carbon monoxide (CO), total alkyl nitrates (ΣAN) and the surrogate species NO z (=NO y - NO x ) revealed that the potential for ozone production increased with plume age. We used NMHC ratios to estimate photochemical ages of the observed biomass burning plumes between 0 and 15 days. Ozone production, calculated from ΔO 3 /ΔCO enhancement ratios, increased from 0.020 ± 0.008 ppbv ppbv −1 in plumes with photochemical ages less than 2 days to 0.55 ± 0.29 ppbv ppbv −1 in plumes with photochemical ages greater than 5 days. In comparing ozone mixing ratios with components of the NO y budget we observed that plumes with ages between 2 and 4 days were characterised by high aerosol loading, relative humidity greater than 40%, and low ozone production efficiencies of 8 ppbv ppbv −1 relative to ΣAN and 2 ppbv ppbv −1 relative to NO z . In plumes with ages greater than 4 days, ozone production efficiency increased to 473 ppbv ppbv −1 relative to ΣAN and 155 ppbv ppbv −1 relative to NO z . From the BORTAS measurements we estimated that aged plumes with low aerosol loading were close to being in photostationary steady state and ozone production in younger plumes was inhibited by high aerosol loading and greater production of ΣAN relative to ozone. The BORTAS measurements of ozone photochemistry in boreal biomass burning plumes were found to be consistent with previous summertime aircraft measurements made over the same region during the Arctic Research of the Composition of the troposphere (ARCTAS-B) in 2008 and Atmospheric Boundary Layer Experiment (ABLE 3B) in 1990.

Description: Gravitational separation in the stratosphere – a new indicator of atmospheric circulation Atmospheric Chemistry and Physics Discussions, 13, 4839-4861, 2013 Author(s): S. Ishidoya, S. Sugawara, S. Morimoto, S. Aoki, T. Nakazawa, H. Honda, and S. Murayama As a basic understanding of the dynamics of the atmospheric circulation, it has been believed that the gravitational separation of atmospheric components is observable only in the atmosphere above the turbopause. However, we found, from our high-precision measurements of not only the isotopic ratios of N 2 , O 2 and Ar but also the concentration of Ar, that the gravitational separation occurs even in the stratosphere below the turbopause; their observed vertical profiles are in good agreement with those expected theoretically from molecular mass differences. The O 2 /N 2 ratio observed in the middle stratosphere, corrected for the gravitational separation, showed the same mean air age as estimated from the CO 2 concentration. Simulations with a 2-dimensional model of the middle atmosphere indicated that a relationship between the gravitational separation and the air age in the stratosphere would be significantly affected if the Brewer-Dobson circulation is enhanced due to global warming. Therefore, the gravitational separation is usable as a new indicator of changes in the atmospheric circulation in the stratosphere.

Description: Spatial and seasonal distribution of Arctic aerosols observed by CALIOP (2006–2012) Atmospheric Chemistry and Physics Discussions, 13, 4863-4915, 2013 Author(s): M. Di Pierro, L. Jaeglé, E. W. Eloranta, and S. Sharma We use retrievals of aerosol extinction from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on board the CALIPSO satellite to examine the vertical, horizontal and temporal variability of tropospheric Arctic aerosols during 2006–2012. We develop an empirical method that takes into account the difference in sensitivity between daytime and nighttime retrievals over the Arctic. Comparisons of the retrieved aerosol extinction to in situ measurements at Barrow (Alaska) and Alert (Canada) show that CALIOP reproduces the observed seasonal cycle and magnitude of surface aerosols to within 25%. In the free troposphere, we find that daytime CALIOP retrievals will only detect the strongest aerosol haze events as demonstrated by a comparison to aircraft measurements obtained during NASA's ARCTAS mission during April 2008. This leads to a systematic underestimate of the column aerosol optical depth by a factor of 2–10. However, when the CALIOP sensitivity threshold is applied to aircraft observations, we find that CALIOP reproduces in situ observations to within 20% and captures the vertical profile of extinction over the Alaskan Arctic. Comparisons with the ground-based HSRL Lidar at Eureka, Canada, show that CALIOP and HSRL capture the evolution of the aerosol backscatter vertical distribution from winter to spring, but a quantitative comparison is inconclusive as the retrieved HSRL backscatter appears to overestimate in situ observations factor of 2 at all altitudes. In the High Arctic (〉 70° N) near the surface ( 〈 2 km), CALIOP aerosol extinctions reach a maximum in December-March (10–20 Mm −1 ), followed by a sharp decline and a minimum in May–September (1–4 Mm −1 ), thus providing the first Pan-Arctic view of Arctic Haze seasonality. The European and Asian Arctic sectors display the highest wintertime extinctions, while the Atlantic sector is the cleanest. Over the Low Arctic (60–70° N) near the surface, CALIOP extinctions reach a maximum over land in summer due to boreal forest fires. During summer, we find that smoke aerosols reach higher altitudes (up to 4 km) over Eastern Siberia and North America than over Northern Eurasia, where it remains mostly confined below 2 km. In the free troposphere, the extinction maximum over the Arctic occurs in March–April at 2–5 km altitude and April–May at 5–8 km. This is consistent with transport from the mid-latitudes associated with the annual maximum in cyclonic activity and blocking patterns in the Northern Hemisphere. A strong gradient in aerosol extinction is observed between 60° N and 70° N in the summer. This is likely due to efficient stratocumulus wet scavenging at high latitudes combined with the poleward retreat of the polar front. Interannual variability in the middle and upper troposphere is associated with biomass burning events (high extinctions observed by CALIOP in spring 2008 and summer 2010) and volcanic eruptions (Kasatochi in August 2008 and Sarychev in June 2009). CALIOP displays below-average extinctions observed from August 2009 through May 2010, which appear to be linked with a strongly negative Arctic Oscillation index.

Description: Filamentary structure in chemical tracer distributions near the subtropical jet following a wave breaking event Atmospheric Chemistry and Physics Discussions, 13, 5039-5089, 2013 Author(s): J. Ungermann, L. L. Pan, C. Kalicinsky, F. Olschewski, P. Knieling, J. Blank, K. Weigel, T. Guggenmoser, F. Stroh, L. Hoffmann, and M. Riese This paper presents a set of observations and analyses of trace gas cross-sections in the extratropical upper troposphere/lower stratosphere (UTLS). The spatially highly-resolved (≈0.5 km vertically and 12.5 km horizontally) cross-sections of ozone (O 3 ), nitric acid (HNO 3 ), and peroxyacetyl nitrate (PAN), retrieved from the measurements of the CRISTA-NF infrared limb sounder flown on the Russian M55-Geophysica, revealed intricate layer structures in the region of the subtropical tropopause break. The chemical structure in this region shows an intertwined stratosphere and troposphere. The observed filaments in all discussed trace gases are of a spatial scale of less than 0.8 km vertically and about 200 km horizontally across the jet-stream. Backward trajectory calculations confirm that the observed filaments are the result of a breaking Rossby wave in the preceding days. An analysis of the trace gas relationships between PAN and O 3 identifies four distinct groups of air mass: polluted subtropical tropospheric air, clean tropical upper-tropospheric air, the lowermost stratospheric air, and air from the deep stratosphere. The tracer relationships further allow the identification of tropospheric, stratospheric, and the transitional air mass made of a mixture of UT and LS air. Mapping of these air mass types onto the geo-spatial location in the cross-sections reveals a highly structured extratropical transition layer (ExTL). Finally, the ratio between the measured reactive nitrogen species (HNO 3 + PAN + ClONO 2 ) and O 3 is analysed to estimate the influence of tropospheric pollution on the extratropical UTLS. In combination, these diagnostics provide the first example of a multi-species two-dimensional picture of a chemically inhomogeneous UTLS region. Since Rossby wave breaking occurs frequently in the region of the tropopause break, these observed fine scale filaments are likely ubiquitous in the region. The implications of the layered structure for chemistry and radiation need to be examined, and the representation of this structure in chemistry-climate models is discussed.

Description: Aerosol physical and chemical properties retrieved from ground-based remote sensing measurements during heavy haze days in Beijing winter Atmospheric Chemistry and Physics Discussions, 13, 5091-5122, 2013 Author(s): Z. Q. Li, X. Gu, L. Wang, D. Li, K. Li, O. Dubovik, G. Schuster, P. Goloub, Y. Zhang, L. Li, Y. Xie, Y. Ma, and H. Xu With the development of economy in the past thirty years, many large cities in the eastern and southwestern China are experiencing increased haze events and atmospheric pollution, causing significant impacts on the regional environment and even climate. However, knowledge on the aerosol physical and chemical properties in heavy haze conditions is still insufficient. In this study, two winter heavy haze events in Beijing occurred in 2011 and 2012 were selected and investigated by using the ground-based remote sensing measurements. We used CIMEL CE318 sun-sky radiometer to derive haze aerosol optical, physical and chemical properties, including aerosol optical depth (AOD), size distribution, complex refractive indices and fractions of chemical components like black carbon (BC), brown carbon (BrC), mineral dust (DU), ammonium sulfate-like (AS) components and aerosol water content (AW). The retrieval results from a total of five haze days showed that the aerosol loading and properties during the two winter haze events were relatively stable. Therefore, a parameterized heavy haze characterization was drawn to present a research case for future studies. The averaged AOD is 3.2 at 440 nm and Ångström exponent is 1.3 from 440–870 nm. The coarse particles occupied a considerable fraction of the bimodal size distribution in winter haze events, with the mean particle radius of 0.21 and 2.9 μm for the fine and coarse mode respectively. The real part of the refractive indices exhibited a relatively flat spectral behavior with an average value of 1.48 from 440 to 1020 nm. The imaginary part showed obviously spectral variation with the value at 440 nm (about 0.013) higher than other three wavelengths (e.g. about 0.008 at 675 nm). The chemical composition retrieval results showed that BC, BrC, DU, AS and AW occupied 1%, 2%, 49%, 15% and 33% respectively on average for the investigated haze events. The comparison of these remote sensing results with in situ BC and PM 2.5 measurements were also presented in the paper.

Description: Pauci ex tanto numero : reducing redundancy in multi-model ensembles Atmospheric Chemistry and Physics Discussions, 13, 4989-5038, 2013 Author(s): E. Solazzo, A. Riccio, I. Kioutsioukis, and S. Galmarini We explicitly address the fundamental issue of member diversity in multi-model ensembles. To date no attempts in this direction are documented within the air quality (AQ) community, although the extensive use of ensembles in this field. Common biases and redundancy are the two issues directly deriving from lack of independence, undermining the significance of a multi-model ensemble, and are the subject of this study. Shared biases among models will determine a biased ensemble, making therefore essential the errors of the ensemble members to be independent so that bias can cancel out. Redundancy derives from having too large a portion of common variance among the members of the ensemble, producing overconfidence in the predictions and underestimation of the uncertainty. The two issues of common biases and redundancy are analysed in detail using the AQMEII ensemble of AQ model results for four air pollutants in two European regions. We show that models share large portions of bias and variance, extending well beyond those induced by common inputs. We make use of several techniques to further show that subsets of models can explain the same amount of variance as the full ensemble with the advantage of being poorly correlated. Selecting the members for generating skilful, non-redundant ensembles from such subsets proved, however, non-trivial. We propose and discuss various methods of member selection and rate the ensemble performance they produce. In most cases, the full ensemble is outscored by the reduced ones. We conclude that, although independence of outputs may not always guarantee enhancement of scores (but this depends upon the skill being investigated) we discourage selecting the members of the ensemble simply on the basis of scores, that is, independence and skills need to be considered disjointly.

Description: Changes in particulate matter physical properties during Saharan advections over Rome (Italy): a four-year study, 2001–2004 Atmospheric Chemistry and Physics Discussions, 13, 4963-4988, 2013 Author(s): G. P. Gobbi, F. Angelini, F. Barnaba, F. Costabile, J. M. Baldasano, S. Basart, R. Sozzi, and A. Bolignano Particulate matter mass concentrations measured in the city of Rome (Italy) in the period 2001–2004 have been cross-analysed with concurrent Saharan dust advection events to infer the impact these natural episodes bear on the standard air quality parameter PM 10 observed at two city stations and at one regional background station. Natural events as Saharan dust advections are associated to a definite health risk. At the same time, the Directive 2008/50/EC allows subtraction of PM exceedances caused by natural contributions from statistics used to determine air-quality of EU sites. In this respect, it is important to detect and characterize such advections by means of reliable, operational techniques. To assess the PM 10 increase we used both the "regional-background method" suggested by EC Guidelines and a "local background" one, demonstrated to be most suited to this central Mediterranean region. The two approaches provided results within 20% from each other. The sequence of Saharan advections over the city has been either detected by Polarization Lidar (laser radar) observations or forecast by the operational numerical regional mineral dust model BSC-DREAM8b of the Barcelona Supercomputing Centre. Lidar observations were also employed to retrieve the average physical properties of the dust clouds as a function of height. Along the four-year period, Lidar measurements (703 evenly distributed days) revealed Saharan plumes transits over Rome on 28.6% of the days, with minimum occurrence in wintertime. Dust was observed to reach the ground on 17.5% of the days totalling 88 episodes. Most (90%) of these advections lasted up to 5 days, averaging to ~3 days. Median time lag between advections was 7 days. Typical altitude range of the dust plumes was 0–6 km, with centre of mass at ~3 km a.g.l. BSC-DREAM8b model simulations (1461 days) predicted Lidar detectable (532nm extinction coefficient 〉0.005 km −1 ) dust advections on 25.9% of the days, with ground contacts on 13% of the days. As in the Lidar case, the average dust centre of mass was forecast at ~3 km. Along the 703-day Lidar dataset, model forecast and Lidar detection of the presence of dust coincided on 80% of the cases, 92% coincidences are found within a ±1-day window. Combination of the BSC-DREAM8b and Lidar records leads to about 21% of the days being affected by presence of Saharan dust at the ground. This combined dataset has been used to compute the increase in PM with respect to dust-unaffected previous days. This analysis has shown Saharan dust events to exert a meaningful impact on the PM 10 records, causing average increases of the order of 11.9 μg m −3 . Conversely, PM 10 increases computed relying only on the Lidar detections (i.e., presence of dust layers actually observed) were of the order of 15.6 μg m −3 . Both analyses indicate the annual average contribution of dust advections to the city PM 10 mass concentrations to be of the order of 2.35 μg m −3 . These results confirm Saharan advections in the central Mediterranean as important modulators of PM 10 loads and exceedances.

Description: Immersion freezing of water and aqueous ammonium sulphate droplets initiated by Humic Like Substances as a function of water activity Atmospheric Chemistry and Physics Discussions, 13, 4917-4961, 2013 Author(s): Y. J. Rigg, P. A. Alpert, and D. A. Knopf Immersion freezing of water and aqueous (NH 4 ) 2 SO 4 droplets containing Leonardite (LEO) and Pahokee peat (PP) serving as surrogates for Humic Like Substances (HULIS) has been investigated. Organic aerosol containing HULIS are ubiquitous in the atmosphere, however, their potential for ice cloud formation is uncertain. Immersion freezing has been studied for temperatures as low as 215 K and solution water activity, a w , from 0.85–1.0. The freezing temperatures of water and aqueous solution droplets containing LEO and PP are 5–15 K warmer than homogeneous ice nucleation temperatures. Heterogeneous freezing temperatures can be represented by a horizontal shift of the ice melting curve as a function of solution a w , Δ a w , by 0.2703 and 0.2466, respectively. Corresponding heterogeneous ice nucleation rate coefficients, J het , are (9.6 ± 2.5)×10 4 and (5.4 ± 1.4)×10 4 cm −2 s −1 for LEO and PP containing droplets, respectively, and remain constant along freezing curves characterized by Δ a w . Consequently predictions of freezing temperatures and kinetics can be made without knowledge of the solute type when relative humidity and IN surface areas are known. The acquired ice nucleation data are applied to evaluate different approaches to fit and reproduce experimentally derived frozen fractions. In addition, we apply a basic formulation of classical nucleation theory (α( T )-model) to calculate contact angles and frozen fractions. Contact angles calculated for each ice nucleus as a function of temperature, α( T )-model, reproduce exactly experimentally derived frozen fractions without involving free fit parameters. However, assigning the IN a single contact angle for entire population (single-α model) is not suited to represent the frozen fractions. Application of α-PDF, active sites, and deterministic model approaches to measured frozen fractions yield similar good representations. Thus, from fitting frozen fractions only, the underlying ice nucleation mechanism and nature of the ice nucleating sites cannot be inferred. In contrast to using fitted functions obtained to represent experimental conditions only, we suggest to use experimentally derived J het as a function of temperature and a w that can be applied to conditions outside of those probed in laboratory. This is because J het ( T ) is independent of time and IN surface areas in contrast to the fit parameters obtained by representation of experimentally derived frozen fractions.

Description: Long-term measurements of particle number size distributions and the relationships with air mass history and source apportionment in the summer of Beijing Atmospheric Chemistry and Physics Discussions, 13, 5165-5197, 2013 Author(s): Z. B. Wang, M. Hu, Z. J. Wu, D. L. Yue, L. Y. He, X. F. Huang, X. G. Liu, and A. Wiedensohler A series of long-term and temporary measurements were conducted to study the improvement of air quality in Beijing during Olympic Games period (8–24 August 2008). To evaluate actions taken to improve the air quality, comparisons of particle number and volume size distributions of August 2008 and 2004–2007 were performed. The total particle number and volume concentrations were 14 000 cm −3 and 37 μm 3 cm −3 in August of 2008, respectively. These were reductions of 41% and 35% compared with the mean values of August 2004–2007. A cluster analysis on air mass history and source apportionment were performed, exploring reasons of the reduction of particle concentrations. Back trajectories were classified into five major clusters. Air mass from south direction are always associated with pollution events during the summertime of Beijing. In August 2008, the frequency of air mass arriving from south has been twice higher compared to the average of the previous years, these southerly air masses did however not result in elevated particle volume concentrations in Beijing. This result implied that the air mass history was not the key factor, explaining reduced particle number and volume concentrations during the Beijing 2008 Olympic Games. Four factors were found influencing particle concentrations using a Positive matrix factorization (PMF) model. They were identified to local and remote traffic emissions, combustion sources as well as secondary transformation. The reductions of the four sources were calculated to 47%, 44%, 43% and 30%, respectively. The significant reductions of particle number and volume concentrations may attribute to actions taken, focusing on primary emissions, especially related to the traffic and combustion sources.

Description: OH reactivity in a South East Asian Tropical rainforest during the Oxidant and Particle Photochemical Processes (OP3) project Atmospheric Chemistry and Physics Discussions, 13, 5233-5278, 2013 Author(s): P. M. Edwards, M. J. Evans, K. L. Furneaux, J. Hopkins, T. Ingham, C. Jones, J. D. Lee, A. C. Lewis, S. J. Moller, D. Stone, L. K. Whalley, and D. E. Heard OH reactivity, the reciprocal of its lifetime from reaction with its sinks, was measured for 12 days in April 2008 within a tropical rainforest on Borneo as part of the OP3 project. The maximum observed value was 83.8 ± 26.0 s −1 with the campaign averaged noon-time maximum being 29.1 ± 8.5 s −1 . The maximum OH reactivity calculated using the campaign averaged noon-time concentrations of observed sinks was ~18 s −1 , significantly less than the observations, consistent with other studies in similar environments. OH reactivity was dominated by reaction with isoprene. Numerical simulations of isoprene oxidation using the Master Chemical Mechanism (v3.2) in a highly simplified physical and chemical environment show that the steady state OH reactivity is a linear function of the OH reactivity due to isoprene alone, with a maximum multiplier being equal to the number of isoprene OH attackable bonds (10). Thus the emission of isoprene constitutes a significantly larger emission of reactivity than is offered by the primary reaction with isoprene alone, with significant scope for the secondary oxidation products of isoprene to constitute the missing reactivity. A physically and chemically more sophisticated simulation (including physical loss, photolysis, and other oxidants) showed that the calculated OH reactivity is reduced by the removal of the OH attackable bonds by other oxidants and photolysis, and by physical loss (mixing and deposition). The calculated OH reactivity is increased by peroxide cycling, and by the OH concentration itself. Notable in these calculations is that the lifetime of OH reactivity is significantly longer than the lifetime of isoprene and critically depends on the chemical and physical lifetime of intermediate species. When constrained to the observed campaign averaged diurnal concentrations of primary volatile organic compounds (VOCs), O 3 , nitrogen oxides (NO x ) and other parameters, the model underestimated the observed mean OH reactivity by 30%. However, it was found that: (1) the short lifetimes of isoprene and OH lead to a large variability in their concentrations and so significant variation in the calculated OH reactivity, (2) uncertainties in the OH chemistry in these high isoprene environments can lead to an underestimate of the OH reactivity, and (3) the physical loss of species that react with OH plays a significant role in the calculated OH reactivity, (4) a missing primary source of reactive carbon would have to be emitted at a rate equivalent to 50% that of isoprene to account for the missing OH sink. A clear argument for a significant missing flux of primary emitted VOC compounds to account for the unmeasured reactivity is not found and the development of techniques for the measurement of secondary multifunctional carbon compounds is needed to close the OH reactivity budget.

Description: Evaluation of spatio-temporal variability of Hamburg Aerosol Climatology against aerosol datasets from MODIS and CALIOP Atmospheric Chemistry and Physics Discussions, 13, 5123-5163, 2013 Author(s): V. Pappas, N. Hatzianastassiou, C. Papadimas, C. Matsoukas, S. Kinne, and I. Vardavas The new global aerosol climatology named HAC (Hamburg Aerosol Climatology) is compared against MODIS (MODerate resolution Imaging Spectroradiometer, Collection 5, 2000–2007) and CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization, Level 2-Version 3, 2006–2011) retrievals. The HAC aerosol optical depth (AOD) values are larger than MODIS in heavy aerosol load conditions (over land) and lower over oceans. Agreement between HAC and MODIS is better over land and for low AOD. Hemispherically, HAC has 16–17% smaller AOD values than MODIS. The discrepancy is slightly larger for the Southern Hemisphere (SH) than for the Northern Hemisphere (NH). Seasonally, the largest absolute differences are from March to August for NH and from September to February for SH. The spectral variability of HAC AOD is also evaluated against AERONET (1998–2007) data for sites representative of main aerosol types (pollutants, sea-salt, biomass and dust). The HAC has a stronger spectral dependence of AOD in the UV wavelengths, compared to AERONET and MODIS. For visible and near-infrared wavelengths, the spectral dependence is similar to AERONET. For specific sites, HAC AOD vertical distribution is compared to CALIOP data by looking at the fraction of columnar AOD at each altitude. The comparison suggests that HAC exhibits a smaller fraction of columnar AOD in the lowest 2–3 km than CALIOP, especially for sites with biomass burning smoke, desert dust and sea salt spray. For the region of the greater Mediterranean basin, the mean profile of HAC AOD is in very good agreement with CALIOP. The HAC AOD is very useful for distinguishing between natural and anthropogenic aerosols and provides high spectral resolution and vertically resolved information.

Description: Newly observed peroxides and the water effect on the formation and removal of hydroxyalkyl hydroperoxides in the ozonolysis of isoprene Atmospheric Chemistry and Physics Discussions, 13, 5279-5314, 2013 Author(s): D. Huang, Z. M. Chen, Y. Zhao, and H. Liang The ozonolysis of alkenes is considered to be an important source of atmospheric peroxides, which serve as oxidants, reservoirs of HO x radicals, and components of secondary organic aerosols (SOAs). Recent laboratory investigations of this reaction identified hydrogen peroxide (H 2 O 2 ) and hydroxymethyl hydroperoxide (HMHP). Although larger hydroxyalkyl hydroperoxides (HAHPs) were also expected, their presence is not currently supported by experimental evidence. In the present study, we investigated the formation of peroxides in the gas phase ozonolysis of isoprene at various relative humidities on a time scale of tens of seconds, using a quartz flow tube reactor coupled with the online detection of peroxides. We detected a variety of conventional peroxides, including H 2 O 2 , HMHP, methyl hydroperoxide, bis-hydroxymethyl hydroperoxide, and ethyl hydroperoxide, and interestingly found three unknown peroxides. The molar yields of the conventional peroxides fell within the range of values provided in the literature. The three unknown peroxides had a combined molar yield of ~30% at 5% relative humidity (RH), which was comparable with that of the conventional peroxides. Unlike H 2 O 2 and HMHP, the molar yields of these three unknown peroxides were inversely related to the RH. On the basis of experimental kinetic and box model analysis, we tentatively assigned these unknown peroxides to C2–C4 HAHPs, which are produced by the reactions of different Criegee intermediates with water. Our study provides experimental evidence for the formation of large HAHPs in the ozonolysis of isoprene (one of the alkenes). These large HAHPs have a sufficiently long lifetime, estimated as tens of minutes, which allows them to become involved in atmospheric chemical processes, e.g. SOA formation and radical recycling. These standards are needed to accurately specify HAHPs, although their synthesis is a challenge.

Description: Optical, microphysical, mass and geometrical properties of aged volcanic particles observed over Athens, Greece, during the Eyjafjallajökull eruption in April 2010 through synergy of Raman lidar and sunphotometer measurements Atmospheric Chemistry and Physics Discussions, 13, 5315-5364, 2013 Author(s): P. Kokkalis, A. Papayannis, V. Amiridis, R. E. Mamouri, I. Veselovskii, A. Kolgotin, G. Tsaknakis, N. I. Kristiansen, A. Stohl, and L. Mona Vertical profiles of the optical (extinction and backscatter coefficients, lidar ratio and Ångström exponent), microphysical (mean effective radius, mean refractive index, mean number concentration) and geometrical properties, as well as of the mass concentration of volcanic particles from the Eyjafjallajökull eruption were retrieved at selected heights over Athens, Greece using a multi-wavelength Raman lidar system and inversion models, during 21–24 April 2010. Additionally, Aerosol Robotic Network (AERONET) particulate columnar measurements indicated the presence of volcanic particles over our area. Simulations of the volcanic partilcles dispersion, done by the FLEXPART model, confirmed the presence of these particles over Athens. Our lidar data showed volcanic particles layers, in the form of filaments after 7-day transport from the source (approximately 4000 km away from our site) between from ground levels up to nearly 10 km. Over Athens the volcanic particles layers were found to be mixed with locally produced aerosols, inside the Planetary Boundary Layer (PBL). Mean hourly-averaged lidar signals indicated that the layer thickness of volcanic particles, ranged between 1.5 and 2.2 km. The corresponding aerosol optical depth (AOD) found to vary from 0.014 to 0.184 at 355 nm and from 0.017 up to 0.174 at 532 nm. Furthermore, the corresponding lidar ratios (LR) ranged between 59.7–79.6 sr (at 355 nm) and 43.9–88.3 sr (at 532 nm). Additionally, we calculated that the mean effective radius of the volcanic particles was 0.13–0.38 μm, while their refractive index ranged from 1.39+0.009 i to 1.48+0.006 i . Finally, our data also allowed us to quantitatively compare, for the first time, the volcanic ash concentrations simulated by FLEXPART with those calculated by the inversion code LIRIC, using data sets derived from coincident lidar-AERONET measurements. In general, good agreement was found between simulations and observations, concerning not only the geometrical properties of the volcanic particles layers, but also the particles mass concentration, with a correlation coefficient of the order of 0.75.

Description: Seasonal cycle and modal structure of particle number size distribution at Dome C, Antarctica Atmospheric Chemistry and Physics Discussions, 13, 5729-5768, 2013 Author(s): E. Järvinen, A. Virkkula, T. Nieminen, P. P. Aalto, E. Asmi, C. Lanconelli, M. Busetto, A. Lupi, R. Schioppo, V. Vitale, M. Mazzola, T. Petäjä, V.-M. Kerminen, and M. Kulmala We studied new particle formation and modal behavior of ultrafine aerosol particles on the high Antarctic East-Plateau at the Concordia station, Dome C (75°06' S, 123°23' E). Aerosol particle number size distributions were measured in the size range 10–600 nm from 14 December 2007 to 7 November 2009. We used an automatic algorithm for fitting up to three modes to the size distribution data. The total particle number concentration was low with the median of 109 cm −3 . There was a clear seasonal cycle in the total particle number and the volume concentrations. The concentrations were at their highest during the austral summer with the median values of 260 cm −3 and 0.086 μm 3 cm −3 , and at their lowest during the austral winter with corresponding values of 15 cm −3 and 0.009 μm 3 cm −3 . New particle formation events were determined from the size distribution data. During the measurement period, new particle formation was seen on 80 days and for 15 of these days the particle growth rates from 10 to 25 nm size could be determined. The median particle growth rate during all these events was 2.5 nm h −1 and the median formation rate of 10 nm particles was 0.023 cm −3 s −1 . Most of the events were similar to those observed in other continental locations, yet also some variability in event types was observed. Exceptional features in Dome C were the winter events that occurred during dark periods, as well as the events for which the growth could be followed during several consecutive days. We called these latter events as slowly-growing events. This paper is the first one to analyze long-term size distribution data from Dome C, and also the first paper to show that new particle formation events occur in the central Antarctica.

Description: Atmospheric mercury over sea ice during the OASIS-2009 campaign Atmospheric Chemistry and Physics Discussions, 13, 5687-5728, 2013 Author(s): A. Steffen, J. Bottenheim, A. Cole, T. A. Douglas, R. Ebinghaus, U. Friess, S. Netcheva, S. Nghiem, H. Sihler, and R. Staebler Measurements of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particulate mercury (PHg) were collected on sea ice near open leads in the Beaufort Sea near Barrow, Alaska in March 2009 as part of the Ocean-Atmosphere-Sea Ice-Snowpack (OASIS) International Polar Year Program. These results represent the first atmospheric mercury speciation measurements collected on the sea ice. Concentrations of PHg over the sea ice averaged 393.5 pg m −3 (range 47.1–900.1 pg m −3 ) during the two week long study. RGM concentrations averaged 30.1 pg m −3 (range 3.5–105.4 pg m −3 ). The mean GEM concentration of 0.59 ng m −3 during the entire study (range 0.01–1.51 ng m −3 ) was depleted compared to annual Arctic ambient boundary layer concentrations. It was shown that when ozone (O 3 ) and bromine oxide (BrO) chemistry are active there is a~linear relationship between GEM, PHg and O 3 but there was no correlation between RGM and O 3 . There was a linear relationship between RGM and BrO and our results suggest that the origin and age of air masses play a role in determining this relationship. These results were the first direct measurements of these atmospheric components over the sea ice. For the first time, GEM was measured simultaneously over the tundra and the sea ice. The results show a significant difference in the magnitude of the emission of GEM from the two locations where significantly higher emission occurs over the tundra. Elevated chloride levels in snow over sea ice are believed to be the cause of lower GEM emissions over the sea ice because chloride has been shown to suppress photoreduction processes of Hg(II) to Hg(0) (GEM) in snow. These results are important because while GEM is emitted after depletion events on snow inland, less GEM is emitted over sea ice. Since the snow pack on sea ice retains more mercury than inland snow current models of the Arctic mercury cycle, which are based predominantly on land based measurements, may greatly underestimate atmospheric deposition fluxes. Land based measurements of atmospheric mercury deposition may also underestimate the impacts of sea ice changes on the mercury cycle in the Arctic. The findings reported in this study improve the current understanding of mercury cycling in the changing Arctic. The predicted changes in sea ice conditions and a~more saline snow pack in the Arctic could lead to even greater retention of atmospherically deposited mercury in the future. This could severely impact the amount of mercury entering the Arctic Ocean and coastal ecosystems.

Description: Chemical composition and hygroscopic properties of aerosol particles over the Aegean Sea Atmospheric Chemistry and Physics Discussions, 13, 5805-5841, 2013 Author(s): S. Bezantakos, K. Barmpounis, M. Giamarelou, E. Bossioli, M. Tombrou, N. Mihalopoulos, K. Eleftheriadis, J. Kalogiros, J. D. Allan, A. Bacak, C. J. Percival, H. Coe, and G. Biskos The chemical composition and water uptake characteristics of sub-micrometer atmospheric particles in the region of the Aegean Sea were measured between 25 August and 11 September 2011 in the framework of the Aegean-Game campaign. High time-resolution measurements of the chemical composition of the particles were conducted using an airborne compact Time-Of-Flight Aerosol Mass Spectrometer (cTOF-AMS). These measurements involved two flights from the island of Crete to the island of Lemnos and back. A Hygroscopic Tandem Differential Mobility Analyzer (HTDMA) located on the island of Lemnos was used to measure the ability of the particles to take up water. The HTDMA measurements showed that the particles were internally mixed, having hygroscopic growth factors that ranged from 1.00 to 1.59 when exposed to 85% relative humidity. When the aircraft flew near the ground station on Lemnos, the cTOF-AMS measurements showed that the organic volume fraction of the particles ranged from 43 to 56%. These measurements corroborate the range of hygroscopic growth factors measured by the HTDMA during that time. Good closure between HTDMA and cTOF-AMS measurements was achieved when assuming that the organic species were hydrophobic and had an average density that corresponds to aged organic species. Using the results from the closure study, the cTOF-AMS measurements were employed to determine a representative aerosol hygroscopic parameter κ mix for the whole path of the two flights. Calculated κ mix values ranged from 0.17 to 1.03 during the first flight and from 0.15 to 0.93 during the second flight. Air masses of different origin as determined by back trajectory calculations can explain the spatial variation in the chemical composition and κ mix values of the particles observed in the region.

Description: Contribution from the ten major emission sectors in Europe and Denmark to the health-cost externalities of air pollution using the EVA model system – an integrated modelling approach Atmospheric Chemistry and Physics Discussions, 13, 5871-5922, 2013 Author(s): J. Brandt, J. D. Silver, J. H. Christensen, M. S. Andersen, J. H. Bønløkke, T. Sigsgaard, C. Geels, A. Gross, A. B. Hansen, K. M. Hansen, G. B. Hedegaard, E. Kaas, and L. M. Frohn We have developed an integrated model system, EVA (Economic Valuation of Air pollution), based on the impact-pathway chain, to assess the health-related economic externalities of air pollution resulting from specific emission sources or sectors, which can be used to support policy-making with respect to emission control. Central for the system is a newly developed tagging method capable of calculating the contribution from a specific emission source or sector to the overall air pollution levels, taking into account the non-linear atmospheric chemistry. The main objective of this work is to identify the anthropogenic emission sources in Europe and Denmark that contribute the most to human health impacts using this tagging method. In this study, we applied the EVA system to Europe and Denmark, with a detailed analysis of health-related external costs from the ten major emission sectors and their relative contributions. The paper contains a thorough description of the EVA system, the main results from the assessment of the main contributors and a discussion of the most important atmospheric chemical reactions relevant for interpreting the results. The main conclusion from the analysis of the ten major emission sectors in Europe and Denmark is that the major contributors to health-related external costs are major power production, agriculture, road traffic, and non-industrial domestic combustion, including wood combustion. We conclude that when regulating the emissions of ammonia from the agricultural sector, both the impacts on nature and on human health should be taken into account. This study confirms that air pollution constitutes a serious problem to human health and that the related external costs are considerable. The results in this work emphasize the importance of defining the right questions in the decision making process, since most of the atmospheric chemical compounds are linked via non-linear chemical reactions, which are important to take into account. The results from assessing the impacts from each emission sector depend clearly on the assumption that the other emission sectors are not changed, especially emissions changing concentrations of atmospheric OH and therefore live times of other chemical species.